i am trying find out how you would find the height of a rectangular prism if the surface area is 100 and length is 10 and width is 6. if someone can get me started on the right way it would help ?

Do you mean LATERAL area = 100?

If so:
Al = 2Wh + 2Lh = 100.
2Wh + 2Lh = 100
12*h + 20*h = 100
32h = 100
h = 3.125.