Algebra and Trigonometry 10th Edition

True. In fact, $f(t+nc)=f(t)$, where $n$ is an integer.
$f(t-2c)=f[(t-2c)+c]=f(t-c)$ $f(t-c)=f[(t-c)+c]=f(t)$. So: $f(t-2c)=f(t)$