Find y if the line through (1,y) and (4,5) has a slope of 3

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Find y if the line through (1,y) and (4,5) has a slope of 3.

Slope formula is:
m = (y2 - y1)/(x2 - x1)

With m = 3, we have:
3 = (5 - y)/(4 - 1)
3 = (5 - y)/3

Cross multiply:
5 - y = 9

Subtract 5 from each side
-y = 4

Multiply each side by -1
y = -4