Question

Maribel how did u got this -189=12a+3b
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-84=-7a
12=a

Answer 1

105 = 5A + 3B ......(1)
63 = 4A + 1B ......(2)

105-3*63 = 5A - 3*4A + 3B -3B ... (1)-3(2)
-84 = -7A + 0B
A=-84/-7=12