Solve for a:
[tex]5a+c=-8a\ \ \ |-c\\\\5a=-8a-c\ \ \ |+8a\\\\13a=-c\ \ \ |:13\\\\\boxed{a=-\dfrac{c}{13}}[/tex]
Solve for c:
[tex]5a+c=-8a\ \ \ \ |-5a\\\\\boxed{c=-13a}[/tex]
