c = -6
a = -3/2
m = -24/5
Explanation:
#1
5(c + 2) = -20
5c + 10 = -20
5c = -20 - 10
5c = -30
c = -6
#2
12a + 5 - 8a = -1
4a + 5 = -1
4a = -1 - 5
4a = -6
a = (-6)/4
a = -3/2
#3
m - 3 - 6m = -27
-5m - 3 = -27 → (times -1)
5m + 3 = 27
5m = 27 - 3
5m = 24
m = 24/5