modelling in mechanics edexcel EX 8D

the question

Whats your question?

Question b

Sure, youve posted the model solution so ... ?

Yes but I don’t understand why they have used 180-theta, shouldn’t the answer just be 36.9?

The unit (positive) j vector is the "north" direction so it points up. The acceleration vector 3i-4j points south east (roughly) so the angle between them is > 90. You consider the angle defined when both vectors point away from the angle/intersection
https://www.cuemath.com/geometry/angle-between-vectors/