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