If you're supposed to find the measure of the arc:
The minor arc CD has the same measure as the central angle it subtends. In this case, arc CD has measure 50º. In radians, that's
50º * (π/180 rad/º) = 5π/18 rad
If you instead meant to ask about finding the length of the arc CD, recall that arc lengths and their subtended central angles occur in a fixed ratio equal to the ratio of the circle's total circumference to 2π rad. In other words, if L is the length of the minor arc CD, then
L /(5π/18 rad) = 2π (7 cm) / (2π rad)
==> L = 35π/18 cm
(Notice that arc measure is given radians, while arc length is given in cm, which is why I offer two different answers here.)