A helicopter hovers 40 ft above the ground. Then the helicopter climbs at a rate...

The height of the helicopter at launch is 40. Since this won't change, the height equation will use a constant to represent it.
This can be shown as an additional 21 feet because the helicopter climbs at a speed of 21 feet per second. When t=1, an additional 21 feet is being added to the helicopter's height. When t=2, an additional 42 feet is being added.
Then:
${\displaystyle h=40+21t}$
After 45 seconds:
${\displaystyle h=40+21\left(45\right)}$
${\displaystyle h=40+945}$
${\displaystyle h=985\text{feet}}$