Let the inclination angle of Y=2X be α, then there is TG α = 2, and the slope of L is tg(α+45? )= (tgα+tg45？ )/( 1-tgαtg45？ ) =-3。

Then use a little slant, Y+3 = -3X+6, and tidy it up.

3X+Y-3 = 0。

(Note: The inclination of y = 2x is not equal to 60? . )