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Stone launched vertically upward from top of cliff

1) Luigi launches a stone vertically upward from the top of a cliff. The height of the stone above the base of the cliff is appoximated by the model h=65+10t-5t^2
where t is time in second and h is height in metres

a) how high is the stone above the base of the cliff after 3 seconds?

h = 65 + 10t - 5t^2
h = 65 +10(3) - 5(3)^2
h = 65 +30 - 45
h = 50 m

b) How long does it take for the stone to reach 50m above the base of the cliff?

h = 65 + 10t - 5t^2
h = 65 + 10t - 5t^2
-65 + 50 = 10t - 5t^2
-15 = 10t - 5t^2
can someone help me from here?

c) How long does it take for the stone to hit the ground at the base of the cliff?

I do not know about how to go about completeing this one

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