Answer
$(\frac{a+b}{3}, \frac{c}{3})$
Work Step by Step
$M_{x}$ = $0(m)+0(m)+c(m)$ = $cm$
$M_{y}$ = $a(m)+b(m)+0(m)$ = $(a+b)m$
the total mass of the system is $3m$
so the coordinates of the center of mass are
$(\frac{M_{y}}{M},\frac{M_{x}}{M})$ = $(\frac{(a+b)m}{3m}, \frac{cm}{3m})$ = $(\frac{a+b}{3}, \frac{c}{3})$