The slope-intercept form is y = mx + b (the m representing slope and the b representing y-intercept). Put the points in ratio form as a fraction and subtract the ratio with the smallest numbers from the ratio with the largest ratio (5/1 - 1/-1) to get 4/2. When simplified, it becomes 2/1, so the slope is 2. Using the slope found previously, subtract 1 from the x of (1, 5) and 2 from the y of (1, 5) to get the y-intercept. The resulting point is (0, 3), so the y-intercept is 3. Replace the m with the found slope (2) and b with the found y-intercept (3) to end up with y = 2x + 3.
