How may I derive the hhx curve (hicksian demand curve) for perfect substitutes goods. Lets say U = 2x + y, Px=8, Px'=2, Py=2, M=60. I know that for cobb douglas with u=xy (u=32) y=Px/Py(x). therefore 32=x^2 (px/py) if py=2 we get that 64/px=x^2 therefore x=8/(px)^1/2 = hhx. how to derive the hhx for perfect substitutes?