( 7B - 51A ) / ( 11A + 12B ) = 6
11A + 12B ≠ 0
7B - 51A = 6 * ( 11A + 12B )
7B - 51A = 66A + 72B
66A + 51A = 7B - 72B
117A = - 65B
A = - ( 65/117 ) B
A = - ( 5/9 ) B
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1) A + B = - ( 5/9 ) B + B = 4/9B
2) ( 4/9 )B : ( - 5/9 ) B = - ( 4/5 ) = - 0,8