(a) mtan refers to the slope of the tangent line. Given <em>f(x)</em> = 9 + 7<em>x</em> ², compute the difference quotient:
Then as <em>h</em> approaches 0 - bearing in mind that we're specifically considering <em>h</em> <em>near</em> 0, and not <em>h</em> = 0 - we can eliminate the factor of <em>h</em> in the numerator and denominator, so that
and so the slope of the line at <em>P</em> (0, 9), for which we take <em>x</em> = 0, is 0.
(b) The equation of the tangent line is then <em>y</em> = 9.