Find the derivative of

y=(x^2 + x^3)^4

What I got:
4(x^2+x^3)^3 * (2x+3x^2)

The correct answer:
4x^7(x+1)(3x+2)

How did they get that?

your answer of 4(x^2+x^3)^3 * (2x+3x^2)

is not incorrect, you just didn't go far enough

4(x^2+x^3)^3 * (2x+3x^2)
=4(x^2 + x^3)(x^2 + x^3)(x^2 + x^3)(2x + 3x^2)
= 4(x^2)(1+x)(x^2)(1+x)(x^2(1+x)(x)(2+3x)
= 4x^7 (1+x)^3 (2+3x)

Your "correct" answer is NOT correct