You have to make some assumptions here without a diagram, and those assumptions can be wrong.
The endpoints of <C at the same endpoints as the arc.
The central angle is also 4x
The measure of C is 1/2 the measure of the central angle.
With these in mind we can go on
Equation
2*<C = the measurement of the arc.
2*(x + 5) = 4x Properties of Arcs central angles and interior angles with the vertex and two other points on the legs of angle hitting the circumference.