Today, I am making a triangle-shaped sandbox for my sweet, little Maci. Papa gave me 30' in length of 2"x 4' planks. I would like the longest side to be 12'. What is one possible combination for the other two sides? (show work)

If you have 30' in 2x4 planks and you want the longest side to be 12', subtract 12' from 30' to determine how many feet of planks you have left:

30' - 12' = 18'. You could divide this 18' by 2 and by doing so get two 9' planks for the (equal) shorter sides of your triangular box.