The scatterplot and trend line show a positive correlation between advertising costs and sales. The slope was found using the points (6.4, 117) and (6.6, 120). What is the value of b in the equation of the trend line? y = 15x + b

When x = 6.4, y=117, therefore
15*6.4 + b = 117
b = 117 -  96 = 21

Confirm this value by checking (6.6, 120).
15*6.6 + 21 = 120 (Correct)

Answer: b = 21