The curve C has parametric equations x=2cos(t) and y=3cos(2t). Find and expression for dy/dx in terms of t.

Differentiate both equations given with respect to t.
dx/dt = -2sin(t)
dy/dt = -6sin(2t)

dy/dx = (dy/dt) / (dx/dt)
Sub your values in to get

dy/dx = (3sin(2t))/sin(t)