Answer:
Angle of rotation is 90°.
Step-by-step explanation:
In the given figure point D is rotated about the origin and we get a new point D'
We join D and D' with origin by two lines forming oD and oD'
Now we get slopes of oD as
and oD' as 
Point D and D'are (-1.5, 3.5) and (3.5, 1.5) and origin as (0,0)
= 
= -
= 
= 
Now
×
= 
= - 
Therefore, both the lines oD and oD' are perpendicular to each other.
Angle of rotation is 90°.