let the measure of the angle be 'x', then the measure of its complement is '90-x' and the measure of its supplement is '180-x'

"the measure of the supplement of an angle is 60 degrees less than three times the measure of the complement of the angle" gives the following equation:

(180-x)+60 = 3(90-x)

solving for 'x' we have:

x = 15

the measure of the angle is 15 degrees