No, it should be 2100 = x(25x)(10x + 1), not 2100^3.
To go from here, expand your brackets, set the equation equal to 0 and solve the equation for x using whatever method you prefer.
The competition swimming pool at Saanich Commonwealth Place is in the shape of a rectangular prism and has avolume of 2100 m^3. The dimensions of the pool are x metres deep by 25x metres long by 10x+1 metres wide. What are the actual dimensions of the pool?
The furthest I got was 2100^3 = (x)(25x)(10x+1)?
The answer is 2 m by 21 m by 50 m but I don't know how to do this question.