See A-Z of rehabilitation exercises
In case you cannot see the video, here is a still sequence -
Straight leg raises work and strengthen the quad muscles and the flexion of your hips.
Lie on your back with your head supported on a pillow and your hands by your sides. Bend your good knee to offer support for your hips and back.
Keeping the bad knee straight, slowly lift your leg. Hold it up for a count of 5...
and slowly lower it down again.
Change legs and repeat the exercise, and repeat this 5 times.