A vector field is specified as $E=8xy{a}_x+4(x^2+2)a_y+6z^2{a}_z.$

Given two points, $A(1,2,-1)$ and $B(-2,1,3),$ find:

$($a$)$ The vector $E$ at point A ;

$($b$)$ The unit vector in the direction of $E$ at B ;

$($c$)$ the equation of the surface on which $|E|=60$