see the attached picture to better understand the problem
we know that if the ray is tangent to the circle then diameter ABC is perpendicular to ray FC and the value of (a + b) is equal to 90 degrees
let's check it step 1 find measure arc DC arc DC=180-54-----> 126° ∠DBC=126°------> by central angle step 2 find angle a° we know that
The inscribed angle measures half of the arc it comprises. so ∠a=arc AD/2----> 54/2--->∠a= 27° ∠b=arc DC/2----> 126/2---> ∠b=63° ∠a+∠b=27+63----> 90°------> is ok the answer is a+b is equal to 90 degrees
