$(25$ points$)$ At the instant shown, the bicyclist at $A$ is traveling at $7\frac{m}{s}$ around the curve on the

race track while increasing his speed at $0\mathrm{.}5\frac{m}{s^2}.$ The bicyclist at $B$ is traveling at $8\mathrm{.}5\frac{m}{s}$ along

the straight$-$a$-$way and increasing his speed at $0\mathrm{.}7\frac{m}{s^2}.$ At this instant, calculate:

a$)$ velocity of A with respect to $B$

b$)$ acceleration of A with respect to $B$

$($express your answers as Cartesian vectors$)$