The three inside angles of a triangle need to equal 180 degrees.
Add the 3 equations together and set to equal 180:
7x+4 + 5x+3 + 2x+5 = 180
Simplify by combining the like terms:
14x + 12 = 180
Subtract 12 from each side:
14x = 168
Divide both sides by 14:
x = 168/14
x = 12
