T=3mg (given) at mean position.
For its extreme position lets break its sine and cosine components. sine has no need because we need to find only displacement.
T=3mg(1+cosA)
so,
3mg=3mg(1+cosA)
1=1+cosA
0=cosA
cos90=cosA
A=90
Hence, maximum displacement of the string of the pendulum with respect to the vertical will be at an angle of 90 degree
