the mass of earth is approximately 5.9x10^24 kg, and the mass of venus is approximately 4.9x10^24 kg. find their combined mass

Because they both have the same exponent, just add the numbers together:

5.9 + 4.9 = 10.8

So now you have:

5.9 x 10^24 + 4.9 x 10^24 = 10.8 x 10^24

A scientific number can only have 1 number to the left of the decimal, so you need to move the decimal one place to the left. When you move the decimal to the left you add to the exponent.

The answer is: 1.08 x 10^25