What is the length of a diagonal of a cube with a side length of 1 cm?

With #! i’ve tried to figure it out but i can’t.

Aswimming pool has an input pump for filling the pool and an output pump for emptying the pool. the input pump can fill the pool in 3 hours, and the output pump can drain the pool in 5 hours. as you go to bed, the pool is full, but a neighbor’s kid turns on the output pump. at midnight, you awake to find the pool half empty. immediately, you turn on the input pump, but you are sleepy and forget to turn off the output pump. at what time will the pool become full?

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15