y = mx + b
slope = m = 1/21, so we have
y = 1/2 x + b
The line passes through (2, 5), so we get
5 = 1/2 * 2 + b
5 = 1 + b
b = 4
The equation of the line is
y = 1/2 x + 4
Point (x, 3) has a y-coordinate of 3. We are looking for the x-coordinate.
y = 1/2 x + 4
3 = 1/2 x + 4
-1 = 1/2 x
x = -2
