Question

PLEASE HELP!!!

Answer 1

Y is usage so set y to 0 and solve for x

y = -10x+100

0 = -10x +100
 subtract 100 from each side:
-100 = -10x

divide both sides by -10

x = -100 / -10

x = 10 hours

Answer 2

Using the equation -10x + 100, then the answer has to be 10 hours.

This is because -10 times 10= -100 and -100+100= 0

By the time he uses his phone for 10 hours the battery level will be at 0%.