F varies inversely with the curvature, r. F varies directly with the square of the speed, s. F varies directly with the weight of the car, w.
So your formula should look like this:
Where k is a constant of proportionality. You need to find k, but luckily, you know that the force required to keep a 2200 lb car traveling at 30 mph from skidding on a curve of radius 500 ft is 266lb. So if you plug these into the equation, you can solve for k. Then you will have a relation. From there, just plug in the numbers from the problem you don't know F for, and solve for F.