Find and simplify the difference quotient of f,
f(x + h) - f(x)
h
and h can not equal 0, for the function.
f(x) =
1
7x
for $\displaystyle f(x) = \frac 1{7x}$ we have $\displaystyle f(x + h) = \frac 1{7(x + h)}$
now plug these into the formula and simplify. Hint: multiply by the LCD of the fractions in the numerator over itself. that is, multiply by $\displaystyle \frac {7x(x + h)}{7x(x + h)}$