Normally, you learn "function notation" well before starting Calculus- maybe it's been too long! If f(t)= t^3- 3t, then f(x)=x^3-3x.
You are told that
f(t)= \int_1^{t^2} \frac{\sqrt{1+u^4}{u}du
so
f(x)= \int_1^{x^2}\frac{\sqrt{1+ u^4}{u}du
Use the "Fundamental Theorem of Calculus" to differentiate that.