x^a . x^b = x^(a+b)
Since the primitive is the "opposite" of the derivative, d/dx f(x) = x^(a+b)
Since your derivative has degree a+b, primtive has degree a+b+1
Thus if f(x) = x^(a+b+1), the derivative of f(x) is (a+b+1) . x^(a+b)
Therefore to get rid of the (a+b+1) at the front, let f(x) = (x^(a+b+1)) / (a+b+1)