interior angles of a triangle add upto 180°.
(7x+10)+(x+20)+2x=180°
7x+10+x+20+2x=180
collecting like terms;
7x+x+2x+10+20=180
10x+30=180
10x=180-30
10x=150
10x/10=150/10
x=15°
substituting X in the angles;
7x+10=7(15)+10=115°
x+20=15+20=35°
2x=2(15)=30°
there the largest angle is 7x+10=115°