Answer
$1.5m/s^2$ downward
Work Step by Step
We know that
$F=ma$
$\implies ma=-21lb$
$\implies a=\frac{-21lb}{m}$
$a=\frac{-21lb}{\frac{W}{g}}$
$a=(\frac{-21lb}{W})g$
We plug in the known values to obtain:
$a=(\frac{-21lb}{142lb})(9.8)$
$a=-1.5m/s^2$
The negative sign shows that the direction of acceleration is downward.