At ashley's nursery, there are 12 rows of trees. in each row, there are 21 pine trees and 7 spruce trees. how many of each type of tree are there in all?

So what you have to do is:

To find the number of pine trees:
12 (because there are 12 rows) × 21 (as there are 21 pine trees in each row) , so
12 × 21 = 252
There are 252 pine trees

You then repeat this process to find out how many spruce trees there are
So:
12 (the amount of rows) × 7 (there are 7 spruce trees in each row)
12 × 7 = 84

Number of pine trees = 252
Number of spruce trees = 84