Please explain how to do this, I am very lost

Impoundment type) Hydropower relies on catching rainwater at higher elevations into creeks and rivers, which can then be impounded in a reservoir. The annual hydropower capacity () is linearly related to mass of rain (=) (hence catchment area, and rainfall, ) as well as the average elevation () of the catchment area. Determine the annual rainfall (in mm) for a hydroelectric dam, which generates 1 TWh of electricity per year if it has a catchment area of and average elevation of .

We can assume that we can convert all potential energy in the rain into electricity. Average elevation of the catchment area is not the same as the hydraulic head of a dam. Density of rainwater is and the gravitational acceleration is .

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