Show that if a 0 and b 0, then there exit numbers a and such that aex

Show that if a ≠ 0 and b ≠ 0, then there exit numbers a and β such that aex + be – x equals either a sinh(x + β) or a cosh (x + β). In other words, almost every function of the form f(x) – aex + be – x is a shifted and stretched hyperbolic sine or cosine function.