Find the equation of the tangent plane at the given point p. $f(x, y)=5 x^{2}+8 y^{2}, p=(2,1,28 $ $2 x+y+28 z=31$ $20 x+16 y-z=34$ $z-20 X-16 y=-28$ $2 x+y+28 z=1$ SP.JG.018