if a rectangular prism has a surface area of 408 and its length is 6 and width 9 and a height of y what is y?

The base has a surface area of 54......so the rest of the surface area is  408 - 54 =  354

So

2h(9 + 6)  = 354    divided both sides by 2

h(15)  = 177      divide both sides by 15

h = 177/15  =  11.8 units