Jason L. has provided an excellent answer. He asumes that, by the nature of this question, the ideas of slopes of lines, and the fact that perpendicular lines (2 lines meeting at a right angle) will have slopes that are negative reciprocals of one another, are known to you in the course you're currently taking.
I recommend paying close attention to definitions (such as perpendicularity) as new topics come up throughout your math course. Observe that he's used a formula for the equation of a line -- the slope-intercept form -- and has substituted one point's x & y coordinates so that he could resolve the b of y=mx + b. Then he substituted the coordinates (s,7) into that equation (a different point)--this time, that enabled solving for the unknown value s. That's the kind of awareness required in your course.