Q: Sketch the graph of the curve: r=3 cosθ ?

Whenever plotting the graph of a function it's best to start by calculating some values of the function, in this case values for r given various values for theta. I suggest you determine values for r given theta = 0, pi/6, pi/4, pi/3, pi/2, 2/3 pi, 3/4 pi, 5/6 pi, pi, etc. up to 2pi. Then plot the points in polar coordinates.