Answer
$HB=-0.00038742t^4+0.0579324t^3-1.34723t^2-12.7354t+1385.72$
Work Step by Step
From part a, we have:
$HB=-0.38742t^4+57.9324t^3-1347.23t^2-12735.4t+1385720$
Divide by $1000$:
$HB=-0.00038742t^4+0.0579324t^3-1.34723t^2-12.7354t+1385.72$