Her sister can do 26 push ups. The equation used would be 42=2x-10 because the amount of push ups Lori can do (42) is equal to twice the amount her sister can do (x) minus 10.
When stone hits the ground, it's height will be zero, and since we're finding the time that's required for the stone to hit the ground, we can set h = 0 and solve for t.