You have the following system of equations:
3x + y = 7 (1)
-x + 2y = 0 (2)
In order to solve the previous system by elimination, you proceed as follow:
first, multiply the second equation by 3:
(-x + 2y = 0)(3)
-3x + 6y = 0
next, you sum the last equation with the first equation
3x + y = 7
-3x + 6y = 0
0 + 7y = 7
From the last equation, you solve for y:
7y = 7 divide by 7 both sides
y = 7/7
y = 1
next, you replace the value of y in one of the equation of the system, say in the equation (2) and solve for x:
-x + 2y = 0
-x + 2(1) = 0
-x + 2 = 0 add 2 both sides
x = 2
Hence, the solution to the given system of equations is:
x = 2
y = 1