Create an equation. Since all of the angles of any triangle add up to 180 degrees, our equation will be: 7x+25+9x+25+5x+25=180. Now, jus solve for x. Combine like terms 21x+75=180 Subtract 75 on both sides to further simplify. 21x=105 Finally, divide both sides by 21 to isolate x. X=5 Hope this helps!