In the cylinder below, the radius, r, is 6 m, and the volume is 252m3. What is the altitude of the cylinder?

There is no 'cylinder below'. But I think you've told us enough about it

so that we can answer your question.

The volume of a cylinder that's standing up is

V = (pi) x (radius) ² x (altitude).

Fill in the numbers we know: 252 m³ = (pi) x (6 m) ² x (altitude)

Divide each side by (pi x 36 m²) : (252 m³) / (pi x 36 m²) = altitude

7 / pi m = altitude

2.228 m = altitude (rounded)