Answer: y = -2x+3
Step-by-step explanation
The two marked points on the blue line are (0,3) and (5,-7)
Use the slope formula.

The slope is -2.
A slope of -2 or -2/1 means "down 2, right 1".
Following this motion pattern has us go from (0,3) to (1,1) to (2,-1) to (3,-3) to (4,-5) to (5,-7) and so on.
The y intercept is 3 as this is where the blue line crosses the y axis.
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We found the following
m = -2 = slope
b = 3 = y intercept
The template y = mx+b updates to y = -2x+3 which is the final answer.