Why Does L+ {f(−at)} Equal Zero for a Causal Signal?

[thedc]

Homework Statement

Let f(t) denote a causal signal. Show that L+ {f(−at)} = 0 for any a > 0.

Homework Equations

The Attempt at a Solution

I know that the laplace{f(at)} = 1/a*F(s/a)

but by simply placing -a whenever determining the Laplace, my signs gets confused within the integral. So my answer is

1/-a*F(s/a) and not 0

Does anyone know how to do this so the L+ {f(−at)} = 0 ?

 

[LCKurtz]

What is a "causal signal"?

What does L+ {f(−at)} mean?