They form a straight angle, so they are supplementary. A straight angle is 180 degrees. The two angles add to 180 degrees.
(3x+25)+(2x) = 180
3x+25+2x = 180
(3x+2x) + 25 = 180
5x+25 = 180
5x+25-25 = 180-25 .... subtract 25 from both sides
5x = 155
5x/5 = 155/5 .... divide both sides by 5
x = 31
