Question

2x + 4y = 12 y = x – 3 What is the solution to the system of equations? (–1, 8) (8, –1)

Answer 1

we have

$2x+4y=12$ ------> equation A

$y=x-3$------> equation B

Substitute equation B in equation A

$2x+4[x-3]=12$

solve for x

$2x+4x-12=12$

combine like terms

$6x=24$

$x=4$

Find the value of y

$y=x-3$

$y=4-3=1$

therefore

The answer is

the solution to the system of equations is the point $(4,1)$

Answer 2

Here, y = x - 3
2x + 4y = 12

Substitute it in the equation, 
2x + 4(x - 3) = 12
2x + 4x - 12 = 12
6x = 12 + 12
x = 24/6
x = 4

Then, y = x-3 = 4-3 = 1
In that case, your solution would be: (4, 1)
But it is not in the option, so you could choose directly by substitution and can check one by one: 

Solution = (8, -1)
Can check in both equations, 
2(8) + 4(-1) = 16 - 4 = 12 = R.H.S

Hope this helps!