Polar equation of a hyperbola?

I am a sopre in high school currently taking precalculus. I am having difficulty figuring out the polar equations of the graphs of conics. The particular problem I am having trouble with right now is finding the polar equation of a hyperbola that has an eccentricity of 4 with a directrix of r=5 sec θ. Please give me an explanation of the answer, not just the answer.