What is the solution set of 4x = y and 2x2 − y = 0

4x=y
2x^2 -y =0  so than we substitute the y value from the first equation inside second equation in place of y we get 

2x^2 -4x =0 than we solve it for x we get 

2x^2 -4x =0
2x(x-2) =0

2x =0

x_1 = 0

x - 2 =0

x_2 = 2 

so for x_1 = 0 result y_1 = 4*0 = 0 so the solution set will be (0,0)
and in case of x_2 = 2 result y_2 = 4*2 = 8 so the sol. set will be (2,8)

hope this will help you