Please explain as well

Let $x$:$R^2{R}^3$ be the parametrized surface given by

$x(s,t)=(s^2-t^2,s+t,s^2+3t)$

a$.$ Determine a normal vector to this surface at the point

$(3,1,1)=x(2,-1)$

b$.$ Find an equation for the plane tangent to this surface at the point $(3,1,1).$