Compute and simplify the difference quotient [f(x + h) - f(x)]/h.
f(x) = 1x2 + 6x - 14

\frac{(x+h)^{2}+6(x+h)-14-(x^{2}+6x-14)}{h}

Now, simplify. When you're finished, you should have the derivative of x^{2}+6x-14. Which is 2x+h+6. If h=0, then you have the derivative of f(x).