When data about the sale price of an artist's sketches and the wind speed on the day the sketch was drawn are entered into statistical software, and the software is asked for a regression line, the software provides an (Intercept) value of 30, a wind speed value of 4.1, and a correlation value of r = 0.82.

a. Write the equation for the regression line.

b. Use your equation to estimate the sale price of a sketch drawn on a day when the wind was blowing at 3 miles per hour.

c. Calculate R² for this model. Do you think it is a useful model? Why or why not?