Considering the angle a by cosine rule 11^2 =7 ^2 +15^2 - 2(7)(15)cos(a) When you do the maths, Cos(a) =153/210 =0.729 a= cos inverse of 0.729 a=43 degrees
Considering angle b 7^2=15^2 +11^2 -2(11)(15) cos(b) This will result in cos(b) =297/330=0.9 b= cos inverse of 0.9 = 25.8 degrees
Considering angle c 15^2=7^2 +11^2 - 2(11)(7) cos(c) Cos(c) will be = -55/154 = -0.357 c= cos inverse of -0.357=110.9
Comparing the angles a,b and c, C is the largest size in the triangle with an angle of 110.9 degrees