$f '\left( x \right)=-2x$
To find the slope $m$ of the tangent line at $\left( 3, 4\right)$, find the value of the derivative for $x=3$$f '\left( 3 \right)=-6$
The slope $m$ of the tangent line at $\left( 3, 4\right)$ is $-6$$m=-6$
Write the tangent line in point-slope form using $\left( 3, 4\right)$ and $m=-6$$y-4=-6\left( x-3 \right)$
Rewrite the equation in slope-intercept form$y=-6x+22$