The total amount of degrees in a triangle is 180. All three angles should add up to 180˚
We know that 40+60 = 100
The next step is to do 180-100 to get the other angle.
180-100 = 80
So the measure of the third angle is 80˚
Hope this helps!
(Also if you want to do this in one equation, you would do 40+60+x = 180
40+60+x = 180
100+x = 180 (combine like terms)
x = 80 (Subtract 100 from both sides)