I'll give you steps instead...
1. Let P represent (a sec theta, b tan theta)
2. Find dy/dx, and sub P into to find gradient of PT. Find equation of PT
3. Find equation of PS, using S as the point, and -1/m as the gradient, where m is the gradient of PT.
4. Solve equations from (2) and (3) simulateneously to find co-ordinates of T.
5. Eliminate the parameter (theta) by substitution into given result or otherwise.