26-{\rm kg} rock is thrown vertically upward from the top of a cliff that is 31 {\rm m} high. When it hits the ground at the base of the cliff, the...

A 0.26-{rm kg} rock is thrown vertically upward from the top of a cliff that is 31 {rm m} high. When it hits the ground at the base of the cliff, the rock has a speed of 31 {rm m/s}. Assuming that air resistance can be ignored, find the initial speed of the rock.