Edward C. answered 04/09/20
Caltech Grad for math tutoring: Algebra through Calculus
Why don't you use the Power Rule to integrate f(x) and then set x=1 to determine the constant of integration?
For example, if f(x) was x5 + 4x(1/2) then the integral would be
F(x) = x6/6 + 8x(3/2)/ 3 + C
and using the condition F(1) = -6 gives
1 + 8/3 + C = -6 ==> C = -36/6 - 17/6 = -53/6, so
F(x) = x6/6 + 8x(3/2)/ 3 - 53/6