Quant Query- Questions on Gregmat

What is the equation of the line that passes through both intersection points of the parabolas x^2 = 4y and y^2 = 4x?

Answer Choices:
A. x=4y
B. y=4x
C. y=x
D. y=x+4
E. xy=4

I understood Greg’s method but I would like to know why what I did is wrong. Here’s my approach:

I created two equations
x^2- 4y= 0
y^2- 4x= 0

and then when I tried solving these two equations I finally got x+y= - 4 which is not even one of the options.

Where did I go wrong?

Substitute the value of one variable in other eqn so that you can get a equation with only one isolated variable.