Here's another question that I'm stuck with (again). Can someone help me please?

Q: P is any point on the hyperbola with equation (x^2)/a^2-(y^2)/b^2=1, S is the focus(ae,0) and S' is the focus(-ae,0). Show that

SP-S'P(with the modulus sign) =2a

I'm not really sure where to start, since my usual route of attack, differentiation, has done me no good with this question as it's not related to tangents and normals.

Help would be greatly appreciated.