Of course, it can be solved, but I don't know the theory or the finite steps that need to be taken to get there. 1st, your function statements have 3 variables, but you're only using one (U1).
F1(T,U1,Y2) = U1
F2(T,U1,Y2) = (-M*G-C*U1**2)/M
2nd, all of the other FORTRAN looks correct, but I suggest another computer language to solve the problem, if you know any, like, C, MatLab, or Pascal.