Since the plane descends 1,300 feet per minute it would descend 19,500 feet after 15 minutes (1,300 x 15 = 19,500) Subtract this from the plane's initial altitude 32,000 and the new altitude will be 12,500. (32,000-19,500=12,500)
All you need to do is plug in 3 wherever you see an x! y = 3(3) - 2(3)^2 + 5(3)^3 y = 9 - 2(9) + 5(27) y = 9 - 18 + 135 y = -9 + 135 y = 126 Hope this helps!