motion problem

initial velocity, v_o = 400 mph = 586.67 fps
 

Part (a); final velocity, v = 250 mph = 366.67 fps
$v = v_o + at$

$366.67 = 586.67 - 10.7t$

$t = 20.56 ~ \text{s}$       answer
 

Part (b); t = 4 s
$v = v_o + at$

$v = 586.67 - 10.7(4)$

$v = 543.87 ~ \text{fps} = 32\,632.2 ~ \text{fpm} = 370.82 ~ \text{mph}$       answer