derivative(slope): 2(x-2)+2(y-4)(y')=0
y'=-(x-2)/(y-4)
so at (-15,4) slope(y') = -(-15-2)/(4-4)=17/0
therefore the line is vertical (infinite rise over no run)
so the tangent line is a vertical line at the given y coordinate,
so the line would be x=-15