Finding Surface Area In Exercises $17-20,$ find the area of the surface.

$17.$ The portion of the plane $z=12-3x-2y$ in the first octant

$18.$ The portion of the paraboloid $z=16-x^2-y^2$ in the first octant

$19.$ The portion of the sphere $x^2+y^2+z^2=25$ inside the cylinder $x^2+y^2=9$

$20.$ The portion of the cone $z=2\sqrt[\phantom{2}]{x^2+y^2}$ inside the cylinder $x^2+y^2=$

can you solve for $18?$