Convolution area property derivation

[mafra]

Homework Statement

Let y(t) be the convolution of x(t) with h(t), show that the area under y(t) is the product of the areas under x(t) and h(t)

Homework Equations

Convolution definition

The Attempt at a Solution

I found a derivation but it skips a step, uploaded it here:
htt p://i50.tiny pic.com/s15dus.jpg

 

[Bohrok]

What's the step that you were wondering about?

 

[mafra]

I don't get how it is possible to separate the integrals in "exchanging the order of integration"