Question

(x + 3)^2/3 + 2(x + 3)^1/3 - 15 = 0

Answer 1

try solving
\[z^2+2z-15=0\] and then replace z by
\[\sqrt[3]{x+3}\]

Answer 2

rename (x+3)^(1/3) as y
so you have y^2 + 2y -15=0
solve for y
x= y^(-1/3)-3