(a) Allow me to guide you on this one. Sketch a cone shape. At the circular base of the cone, sketch a straight line across through the middle which will be your diameter(most preferably where the slant height meet the circular base). Label it 24 m. From the apex of the cone, drop a straight line which is perpendicular to the diameter meeting it in the middle. Label that line 16 m. Label the line slanting from the top to the bottom x.
(b) The slant height of the roof is 20 m.
x^{2} =12^{2} + 16^{2} =144 + 256
x^{2} =400
x=
√400
x=20m
