Two parallel lines are crossed by a transversal what is the value of a?

For this case, the first thing to do is to observe that the angle between line f and line h equals 63 degrees.
Then, since line f is parallel to line g, then the angle between line h and line g is equal to the angle between line h and line f.
Therefore, we have to:
a = 63º

the value of a is:

a = 63º

by definition, an arc subtended by a central angle of some given measure has the same measure. so .