Answer
$$\boxed{\textbf{See Work Step By Step}}$$
Work Step by Step
Let $$t=x/c$$
or $$x=ct$$
Therefore,
$lower $ $limit = ca/c =a$
and $upper$ $limit = cb/c =b$
$$\int^{cb}_{ca}f(x)dx=\int^{b}_{a}f(ct)d(ct) = c\int^{b}_{a}f(ct)dt = c\int^{b}_{a}f(x)dx $$
We could do this last step because the variable of a definite integral is just a dummy variable
