Find the Directional Derivative Of $f ( x , y ) = 20 \sqrt { x } - y ^ { 3 }$

Multiple Choice

Find the directional derivative of $f ( x , y ) = 20 \sqrt { x } - y ^ { 3 }$ at the point (1, 3) in the direction toward the point (3, 1) .

A) $37 \sqrt { 2 }$
B) $\frac { 37 } { \sqrt { 2 } }$
C) $\sqrt { 2 }$
D) $37$
E) None of these