a+7b=17, a+2b=2
a + 7b = 17
-
a + 2b = 2
__________
( a - a ) + ( 7b - 2b ) = 17 - 2
5b = 15 Divide both sides by 5
b = 15 / 5 = 3
a + 7b = 17
a + 7 * 3 = 17
a + 21 = 17 Subtract 21 to both sides
a + 21 - 21 = 17 - 21
a = - 4