The problem is asking to find c given that-

$c\equiv 13a(\phantom{\rule{1em}{0ex}}\mathrm{mod}\phantom{\rule{thinmathspace}{0ex}}\phantom{\rule{thinmathspace}{0ex}}19)$ and the variable $a\equiv 11(\phantom{\rule{1em}{0ex}}\mathrm{mod}\phantom{\rule{thinmathspace}{0ex}}\phantom{\rule{thinmathspace}{0ex}}19)$.

I've tried to solve this using algebra and using the theorems but I can't seem to work it out.

Any suggestions?

$c\equiv 13a(\phantom{\rule{1em}{0ex}}\mathrm{mod}\phantom{\rule{thinmathspace}{0ex}}\phantom{\rule{thinmathspace}{0ex}}19)$ and the variable $a\equiv 11(\phantom{\rule{1em}{0ex}}\mathrm{mod}\phantom{\rule{thinmathspace}{0ex}}\phantom{\rule{thinmathspace}{0ex}}19)$.

I've tried to solve this using algebra and using the theorems but I can't seem to work it out.

Any suggestions?