How is this solved using trig identities (sum/difference)?

What number should be added to both sides of the equation to complete this square? x^2-10x=7

For one history test, keith had to answer 40 questions. of these 40 questions, keith answered 28 of them correctly. what percent did keith get on his history test? round your answer to the nearest tenth if necessary

If angle 2=3x +1 and angle 3=2x +4what is angle 2