first you want to line those equations so you can understand them better,
-3A = 36
10A + 3C = 9
2B + 5C = 23
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divide the first equation by -3 and you will get, A =-12
then you use the -12 instead of A in the second equation and you get
10(-12) + 3C = 9, -120 +C = 9, you take -120 to the other side and it would be like this,
3C = 120 + 9, then you sum 120 and 9 and divide the equation by 3, C = 129/3 = 43
and therefore 2B + 5(43) = 23
2B + 215 = 23
2B = -215 +23
2B = -192
B = -96