The problem means that both polynomials have to be the same:
(x^2-4x+3)*(x+5) = (x^2+4x-5)*(x-c)
But they look different. So one has to find all the roots.
x^2-4x+3 = (x-1)(x-3)
x^2+4x-5 = (x-1)(x+5)
So, the problem is indeed find 'c' such that the equality always holds:
(x-1)(x-3)(x+5) = (x-1)(x+5)(x-c),
And, therefore, c=3.
Hope it may still help.