hhere, t gives the time in years since 1950. therefore, if we can find t when the degrees exceed 2,500,000 we can tell the year that it happens, which is the "when" we are looking for.

so plug in D = 2,500,000 and solve the quadratic equation for t

2500000 = -0.0977t^2 + 47.1174t + 291.5651

=> -0.0977t^2 + 47.1174t - 2499708.435 = 0

i suggest using the quadratic formula, do you remember it?