2a + b = 5 inmultim cu 2 => 4a + 2b = 10
2b + 3c = 7 inmultim cu 3 => 6b + 9c = 21 le adunam acum
=> 4a + 8b + 9c = 31
2a+b = 5
2b+3c = 7
4a + 8b +9c = (2a+b) +(2a+b) +(2b+3c) +(2b+3c)+( 2b+3c)
= 5+5+7+7+7
= 10 + 21
= 31
