if x = 5, you would have to substitute 5 into the given statement in place of x whenever x appears. then, you would perform PEMDAS on the new statement to find an answer. (remember that when you are cubing x, you are only cubing x; it would be 6(x⋅x⋅x), not 6x⋅6x⋅6x.
6x^3 + 8x
6(5⋅5⋅5) + 8(5)
750 + 40
= 790