Answer: x + 18
Explanation:
Formula: y = mx + b
m = rise / run or (y2 - y1) / (x2 - x1)
To get m:
x1 = 3, y1 = 21, x2 = 7, y2 = 25 for (3,21) and (7,25).
Now, m = (25 - 21) / (7 - 3) = 4 / 4 = 1
m = 1
To get b, do some algebra with m = 1 and solve for b:
y = 1x + b
b = y - 1x (Subtract 1x from both sides)
Then use either point (3,21) or (7,25) and plug in for x and y. In this case we'll use (3, 21).
b = 21 - 1(3) = 21 - 3 = 18
So, b = 18
Answer y = 1x + 18 or y = x + 18