What is a fitness exercise that I could do to improve my muscular strength?

It's for gym class and we have 45 seconds to do the exercise. Everyone has to be able to do the exercise without to much struggle.What is a fitness exercise that I could do to improve my muscular strength?leg lifts...lie down and cross ur hand and put them under ur butt (so it doesnt hurt) and then lift your legs 6 inchesWhat is a fitness exercise that I could do to improve my muscular strength?arms