There exists a trigonometric identity which states that,
sin (A - B) = sin A cos B - cos A sin B
This is very similar to the given expression with A equal to 57° and B equal to 13°. The simplified form of the angle is,
sin (57° - 13°) = sin 44°
1. Over a period of 6 years (from 1980 to 1986) the house gained a value of 12000 dollars (109k-97k). 12000/6 gives you a rate of 2000 dollars per year. Because the initial price at t=0 is 97000, the function is 97000+2000t