find the equation of tangent to circle at $x^2+y^2-60y-80x+2100=0$

Question

I proceed by assuming parametric coordinate $x=20 \cos t+40,y=20 \sin t+30$. And write equation of tangent at parametric point $(t)$. And then write distance of origin from tangent and minimize distance to solve for $t$. To find the given point but it comes out to be incorrect. What mistake I made.