Lines are described by equations of the form y = mx + b The fast way of finding 'b' is to look for a place where x = 0 and plug in the x and y values into this equation. In the case of this line, when x = 0, y = 5:
5 = m(0) + b 5 = 0 + b 5 = b
To get the slope, 'm', we need to pick two points and divide the difference of their y values by the difference of their x values. Pick points where the line falls on the grid. The one we just used, (0,5), works well. So does (2,1).
Plug into the equation m = (y2-y1)/(x2-x1) to get m = (1-5)/(2-0) = -4/2 = -2
and 
, the product is
and 
, so the product is