Can someone help with this?

Astuntman jumping off a 20-m-high building is modeled by the equation h=20-5t^2, where t is the same in seconds. a high-speed camera is ready to film him between 15m and 10m above the ground. for which interval of time should the camera film him?

Rewrite so the subject can be y in the formula d=m-y/y+n

Gabrielle's age is three times mikhail's age. the sum of their ages is 40 . what is mikhail's age? __ years old