let "a" be the point in

then the point must also be on the tangent line.

so we can now compute the slope using these 2 points

but we also know that the slope must be equal to the derivative evaluated at a so we get

We now have a system of equations in "m" and "a"

setting them equal gives

as far as I know (any input we be awsome )

this doesn't have a closed form solution(there may be another way )

but I solved the equation numericlly with Maple and it gives

this is in radians.

I hope this helps.