0=40-10t-5t^2
t^2+2t-8=0
(t+4)(t-2)=0
t=-4 (not a solution) or t=2
Answer: after 2 seconds
Step-by-step explanation:
I've posted solutions in the picture. Rather the ways to solve them. Check and find answers on your own.
Also, I've not solved the problem by differential calculus. You can, obviously, if you're interested. Use it for ease and for tougher equations.
I don’t know sorry I’m on this app because I need help myself