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Find the slope of the line between the points (3, 15) and (5, 30).
It doesn't matter which way we choose a and b, so we'll do it both ways.
First, take a to be the smaller of the two x values. Here f is the equation of the line. We don't know the equation for f, but we know both points are on the graph of f, which means f(3) = 15 and f(5) = 30.