A sky diver of mass 60kg can slow herself by adjusting her form to a constant speed of 90km/h ... magnitude of the drag force?

If the sky diver is at a constant speed, that means with her current aerodynamics she has reached her terminal velocity. This means that Force due to gravity = Drag, i. e. if you drew her on a free body diagram the downward and upward arrow would have the same magnitude.

Assuming acceleration due to gravity to be 9.81ms^-2, we can work out the force due to gravity on the sky diver (also assuming she is on earth) i. e. her weight, to be 60*9.81 = 588.6N

As she is at a constant velocity, the drag force will be equal to that, since a=f/m where f is the net force acting on the object. Therefore, the magnitude of the drag force is 588.6N