#### Answer

The student accidentally used the x-intercept instead of the y-intercept for b. The correct equation is y=-3x+21.

#### Work Step by Step

The given point was the x-intercept, but the student mistook it for the y-intercept and plugged it in for b.
y=mx+b
0=-3(7)+b
b-21=0
b=21
Therefore, the correct equation is y=-3x+21.