$(3)$ pts$]$ Let $f(x,y)=2x^2+y^2-5y+10.$

$($a$)4pts$ Find $f_x(1,2)$ and $f_y(1,2).$

$($b$)2pts$ Find the gradient gradf$(1,2).$

$($c$)6pts$ Find an equation of the tangent plane to surface $f(x,y)$ at $(1,2,6).$

$($d$)6pts$ Find the directional derivative of $f(x,y)$ at $P(1,2)$ in the direction of the vector