Hello! Formulas in slope-intercept form are y = mx + b. This is where m equals the slope, b equals the y-intercept, and y and x remain unfilled for this problem. The formula for finding the slope is y2 - y1/ x2 - x1, where you subtract the first x and y-coordinates from the second x and y-coordinates. (1, 9) is the first corrdinate and (3, 5) is the second coordinate. Let's start subtracting. 5 - 9 is -4. 3 - 1 is 2. -4/2 is -2. We found the slope, which is -2, but what about the y-intercept? Well, we can find that by putting y as 9, -2 as the slope, and use one of the x values given to do this. It would look like this:
9 = -2(1) + b
We solve for b in this case. Multiply -2 and 1 together to get -2. We add 2 to each side to cancel out -2. You get 11 = b. The y-intercept is 11. So the equation in slope-intercept form is y = -2x + 11
Note: When it comes to finding the y-intercept, you use one of the points, with the x-coordinate you multiply with the slope, and the y-coordinate you plug in as y. Then, you do your addition or subtraction to both sides as you cancel the right side out to get the answer.