Assume 
.
![\dfrac{\mathrm d}{\mathrm dx}[7x^2+xy+7y^2]=\dfrac{\mathrm d}{\mathrm dx}[15]](https://tex.z-dn.net/?f=%5Cdfrac%7B%5Cmathrm%20d%7D%7B%5Cmathrm%20dx%7D%5B7x%5E2%2Bxy%2B7y%5E2%5D%3D%5Cdfrac%7B%5Cmathrm%20d%7D%7B%5Cmathrm%20dx%7D%5B15%5D)
The slope of the tangent line to the curve at
is then 
, so the tangent to 
has slope
The point-slope form of the tangent line is then
The slope of the tangent line to the curve at
The point-slope form of the tangent line is then