You shall use the 2nd Fundamental Theorem of Calculus.

If it was just INT(0,x)f(t)dt

Then the derivative is f(x).

Here it is INT(0,x^2)f(t)dt

Use the chain rule.

The inner function (the "u") is x^2

The other function (the "f(u)") is INT(0,u)f(t)dt.

Thus,

2x*f(u)=2x*f(x^2)