5a-7=4b+6 2b-3=7-4a 3a-6b= ???
5a-7=4b+6 <=> 5a-4b = 13 | * 3/5 <=> 3a -2.4b=7.8 _ 2b-3=7-4a <=> 4a+2b = 10 | * 3 12a + 6b =30 3a -2.4b - 12a-6b=7.8-30 3a - 6b = 12a+2.4b - 22.2