The reason it is C is because with triangles they usually have sidea that add up to c(a^2 + b^2 =c^2). If you add 7 + 11 you realize it matches with 18. And with that all three numbers of answer C is your answer
SO you know that triangles add up to 180. A right angle equals 90 so 180-90=90. And an isosceles triangle means that 2 angles are congruent. SO 90/2 = 45. The measure of the 2 acute angles are 45