At what point on the graph of y= 1/2x^2 is the tangent line parallel to the line 2x-4x=3?

I took the derivative of the first equation, and set it equal to the slope of the second and got that x was equal to 1/2. Then I plugged that back into the first equation to get that the y coordinate would be 1/8. However, the answer states that the y coordinate is -1/2. the only way to get that is to plug x=1/2 into the second equation. why do you use the second equation to find the point on the graph of the first? Thanks!