How do you use subtraction to find a missing addend in an addition problem? explain

If 20% of a number is 35, what is 50% of that number?

After t seconds, a ball tossed in the air from the ground level reaches a height of h feet given by the equation h = 144t-16t^2. after how many seconds will the ball hit the ground before rebounding?

Can someone me with my math problem pls my teacher is on my back about this