To find the equation of a line, we need the slope of the line and a point on the line. since we are requested to find the equation of the tangent line at the point (4, 2), we know that (4, 2) is a point on the line. so we just need to find its slope. the slope of a tangent line to f(x) at x = a can be found using the formula mtan = lim x→a f(x) − f(a) x − a.