[((x^2)+3)^(5/2)-3)]=6

[(x^2 + 3) ^ (5/2 - 3)] = 6

Simplify the exponent.

(x^2 + 3) ^ (-0.5) = 6

Move the exponent to the other side.
x^2 + 3 = 6 ^ -2
x^2 + 3 = 1 / 36
x = ?