Question

2 x A + B =5
2 x B + 3 x C = 7
4 xA+ 8 x B + 9 x C =?

Answer 1

2A+B=5(inmultim cu 2)
4A+2B=10(relatia 1)
2B+3C=7(inmultim cu 3)
6B+9C=21(relatia 2)
adunam relatia 1 cu relatia 2
4A+2B+6B+9C=10+21
4A+8B+9C=31

Answer 2

2a+b =5   
2b + 3c = 7    3c=7-2b   9c=21-6b


4a+ 8b + 9c =4a+8b+21-6b=4a+2b+21 =2(2a+b)+21=2*5+21=31