No, that's not right.
Try it again.
And be careful with the units!
Try changing everything to SI units;
find vi = 70 mi/hr in units of m/s
and x - xi = 186 feet in units of meters.
Anyway, 9.8 m/s2 is "g", the acceleration due to gravity.
a = - g = - 9.8 m/s2 might show up in a free fall problem but
it would be an unlikely coincident for that number to show up here.
(c) 2001, Doug Davis; all rights reserved.