Area of lawn pls don’t send link for a website and explain here

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

What ratio is proportional to 18/21

Carli has 90 apps on her phone. braylen has half the amount of apps as theiss. if carli has three timesthe amount of apps as theiss,how many apps does braylen have? let b represent the number of braylen's apps and t represent the number of theiss's apps.