We know that for an equilateral triangle the orthocentre and centroid coincide.
Here, centroid (h,k)≡(3cost+sint+a,3sint−cost+b)
⇒3h−a=cost+sint...(i)
⇒3k−b=sint−cost...(ii)
Eliminating t from above two equation (i)&(\mathrm{ii}), we get
(h−3a)2+(k−3b)2=92
So, (h,k) lies on the circle whose centre is (3a,3b), now comparing with (1,31) we get,a=3,b=1
Hence a2−b2=8