The function is
.
The equation of tangent line is
.
Slope of the tangent line is
.
Consider
.
Differentiate on each side with respect to
.

Slope of the tangent line is
.

Substitute
in the function.

The tangent line and function are equal at a particular point.
\Hence equate the function and the tangent line equation.
\
Substitute
in the
.

.
.