Final answer:
To find the measure of an unknown angle on a straight line, subtract the sum of the known angles from 180 degrees.
Step-by-step explanation:
The question you asked is about finding the measure of an unknown angle when ABC is a straight angle. A straight angle measures 180 degrees, so if you have one or more angles that are known on a straight line, you can find the unknown angle by subtracting the sum of the known angles from 180 degrees. For instance, if you know one angle is 45 degrees and you are looking for the measure of the unknown angle, you would calculate it by doing 180 - 45 = 135 degrees to find the unknown angle.