To solve this addition problem, we add the digits in each column, starting from the rightmost column. 5 + 4 = 9, 3 + 3 = 6, and 2 + 2 = 4. Therefore, the sum of 345 and 234 is 579.

To find the difference between two numbers, we subtract the smaller number from the larger number. In this case, we have 876 - 345 = 531.

To multiply two numbers, we multiply the digits in each column, starting from the rightmost column. 3 x 2 = 6, 2 x 1 = 2, and 2 x 0 = 0. Then, we add the partial products to get the final product: 6 + 20 + 250 = 276.

To divide two numbers, we repeatedly subtract the divisor from the dividend until the remainder is less than the divisor. In this case, we have 456 ÷ 12 = 38 with a remainder of 0.

The area of a rectangle is calculated by multiplying its length and width. In this case, we have 10 cm x 5 cm = 50 cm².

The perimeter of a square is calculated by multiplying the side length by 4. In this case, we have 8 cm x 4 = 32 cm.

There are 1,000 milliliters in 1 liter. Therefore, to convert 2 liters to milliliters, we multiply 2 by 1,000: 2 x 1,000 = 2,000 ml.

The volume of a cube is calculated by cubing its side length. In this case, we have 4 cm x 4 cm x 4 cm = 64 cm³.

The mean of a set of numbers is calculated by adding all the numbers and dividing the sum by the number of numbers. In this case, we have (12 + 15 + 18 + 21 + 24) ÷ 5 = 90 ÷ 5 = 18.

A rectangle has 4 sides and 4 right angles, while a triangle has 3 sides and 3 angles, a square has 4 sides and 4 right angles, and a circle has no sides or angles.

When flipping a coin, there are two possible outcomes: head or tail. Therefore, the probability of getting a head is 1/2.

To solve this equation, we need to isolate the variable x on one side of the equation. We can do this by subtracting 5 from both sides: 3x + 5 - 5 = 17 - 5, which gives us 3x = 12. Then, we divide both sides by 3: 3x ÷ 3 = 12 ÷ 3, which gives us x = 4.

The area of a circle is calculated using the formula πr², where π is approximately 3.14 and r is the radius of the circle. In this case, we have π x 7² = 3.14 x 49 = 153.86 cm², which is approximately 154 cm².

The volume of a cylinder is calculated using the formula πr²h, where π is approximately 3.14, r is the radius of the cylinder, and h is the height of the cylinder. In this case, we have π x 5² x 10 = 3.14 x 25 x 10 = 785 cm³, which is approximately 314 cm³.

The surface area of a cube is calculated by multiplying the area of one face by 6. In this case, the area of one face is 6 cm x 6 cm = 36 cm². Therefore, the surface area of the cube is 36 cm² x 6 = 216 cm².