how to do? PLS HELP?

Question

by using an appropriate substitution, find the value of x for which x^3/2-8x^-3/2=7

Answer 1

x^3/2 - 8x^-3/2 = 7
let a = x^3/2
then a - 8/a = 7
a^2 - 7a - 8 = 0
(a - 8)(a + 1) = 0
a = -1,8
rejecting x^3/2 = -1,
x^3/2 = 8
x^3 = 64
x = 4

Answer 2

Gotta run, it's late.
Substitute Y for x^(3/2) to get Y-8/Y=7 Y=8,x=4
Good night