Since the rate is constant, the growth of the height of the helicopter follows a linear pattern. The helicopter has an initial quota of 40ft, so that would be the constant term. The linear terms's coefficient is the rate of growth, so 21 ft/s. So, if [tex] t [/tex] is the number of seconds passed from when the observation started, we have
[tex] h(t) = 40+21t [/tex]
Which means that, if [tex] t=45 [/tex], the height is
[tex] h(45)=40+21\cdot45 = 40+945=985 [/tex]
