Focus on the regression line. Notice how (0,90) and (1,75) are on this line. Compute the slope of the line through those points
Use the slope formula
m = (y2-y1)/(x2-x1)
m = (75-90)/(1-0)
m = -15/1
m = -15
So Matthew's balance is decreasing by approximately 15 dollars each month. The negative slope means "decrease". This is shown by the fact that the line goes downhill as you move from left to right.
Answer: decreases by $15 each month