Please help me! 30 points!

What is the missing constant term in the perfect square that starts with x^2 -20x

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

In the rectangle below ac=30 units what is de