You have given that cos theta = -12/5 and we have to determine the values of theta that satisfy the relation.
-12/5 = -2.4
But the cosine of any number lies in the interval [-1, 1]
Therefore there can be no theta such that cos theta = -12/5
It is impossible for a cosine of an angle to be smaller than -1.
We notice that cos `theta` =-12/5 = -2.4 < -1
Therefore, there is no value for `theta` in the given interval (`pi` /2 ; `pi` ), for cos `theta` = -12/5.