Question

A) x+y=17
2x-y=19
B) y=x+13
2x-y=-3

Answer 1

x+y=17
y=17-x

2*(17-y)-y=19
34-2y-y=19
-2y-y=19-34
-3y=-15
y=-15/-3 = 15/3 = 5

x=17-5 = 12 

Answer 2

A)
(1)   x + y = 17
(2)  2x - y = 19

de (1) temos
x = 17 - y

em (2)  2x - y = 19
2(17 - y) - y = 19
34 - 2y - y = 19
-2y -y = 19 - 34
-3y = -15

y = -15/-3
y = 5

x = 17 - y
x = 17 - 5
x = 12

S = (12, 5)