7x^2+1 is a polynomial of degree 2.
x is a polynomial of degree 2-1.
So you have to think : substitution !
So you can't think of the dx/dt...
So you have to substitute by
and the x will simplify :
After that, don't forget to back substitute (you will have a formula with t, but t=7x^2+1, so you have to change it)