What is the equation of the tangent line to f(x)=3-14x at x=8?

Your f(x) is a linear function whose graph is a straight line, so I am not sure whether or not you can find a tangent line to a...line (a tangent line being a line with only one point in common with your curve)! You may argue that a line crossing your line at x=8 (having a point in common) has the same property of the tangent but I think that it doesn't work (also because you'll get an ${\displaystyle \mathrm{\infty }}$ number of them).