Answer:
Definition of reference angle:
The smallest angle that the terminal side of a given angle makes with the x-axis.
Given that the angle theta is
Hence,
To figure out the reference angle, we will have
Assuming a base side for the angle along the X-axis, the reference angle is the smallest acute angle as measured from the X-axis
(yes; I know this is not technically an acute angle, but it is the smallest positive angle).
Therefore,
The reference angle is = 70°