x + y = 8
y = 3
Well, because we know what y equals, we simply plug in 3 for y in the given equation.
x + y = 8
x + (3) = 8
Subtract 3 from both sides.
x = 8 - 3
So, x = 5
(5, 3)
Check work by plugging in x and y in the given equation.
x + y = 8
(5) + (3) = 8
8 = 8
So, our answer is (5, 3)
I am quite sure that you have given us the wrong equations, as none of these answers are correct.
~Hope I helped!~