the area of the triangle = 1/2 * base * altitude = 1.2 * 20 * a = 10a
Work out the altitude (a):-
the altitude forms a right angled triangle with vertex angle = 1/2*108 = 54
so tan 54 = 10 / a
a = 10 / tan 54 = 7.265
area of the triangle = 10 * 7.265 = 72.65 cm^2
