Draw a picture. You will need to show the ladder (draw as a sloping straight line), the wall (a vertical straight line) and the ground (another straight line, assumed horizontal.) Note that only the base of the ladder is 6m from the building. Note also that there is a right angle between the ground and the wall; this is never stated in problems like this but you are expected to know it!

A right angle means that Pythagoras' theorem applies. The sum of the squares of the two short edges of your triangle (one of which you don't know yet) is the square of the ladder length.

Good Hunting!
RD