set the equation
y = a*x + b
1) as the line passes through (0,5), b=5
2) as the line passes through (3,-1), the following equation is established.
-1 = a * 3 + 5
-6 = 3 *a
a = -6/3 = -2
3) we know that a = -2, b=5
thus, the equation of the line is : y = -2x + 5
