Question 37317
Combining like terms means to add the ones that have the same variable(s) and the same power(s).  In this case it would be a b and c.  

for a:  -4a + (-9a) = -13a
for b:  -9b + 3b = -6b
for c there is only one:  +3c

Final equation would be:

-13a - 6b + 3c