The temperature distribution in a fluid is given by $T=12x+7y,$ where $x$ and $y$ are the horizontal and vertical coordinates in meters and $T$ is in degrees centigrade. Determine the time rate of change of temperature of a fluid particle traveling

$($a$)$horizontally with $u=21\frac{m}{s},v=0$;

$($b$)$ vertically with $u=0,v=21\frac{m}{s}.$