With respect to your modified question, things are a little stickier. One thing that might help you in getting the answer is to consider the following representation of $f(t)$:
$f(t)=\int_x^{t^2} \frac{\sqrt{7+u^4}}{u} \, \mathrm du=\int_p^{t^2} \frac{\sqrt{7+u^4}}{u} \, \mathrm du-\int_p^{x} \frac{\sqrt{7+u^4}}{u} \, \mathrm du$
where $p$ is some constant. (Why this is justified is something you'll have to explain.) We then have
$\begin{align*}F(x)=\int_1^x f(t) \, \mathrm dt&=\int_1^x \left(\int_p^{t^2} \frac{\sqrt{7+u^4}}{u} \, \mathrm du-\int_p^{x} \frac{\sqrt{7+u^4}}{u} \, \mathrm du\right) \, \mathrm dt\\&=\int_1^x \int_p^{t^2} \frac{\sqrt{7+u^4}}{u} \, \mathrm du\,\mathrm dt-\left(\int_p^{x} \frac{\sqrt{7+u^4}}{u} \, \mathrm du\right)\left(\int_1^x\mathrm dt\right)\end{align*}$
(You'll also have to explain how I got that last bit.) Differentiating the second term requires that you use the product rule in addition to the Fundamental Theorem; differentiating the first term will require the careful use of the chain rule...