Question 1057802
The definition of the derivative is,
{{{df/dx=(f(x+dx)-f(x))/dx}}} in the limit as dx goes to zero.
So substituting,
{{{t=x+dx}}}
{{{df/dx=(f(t)-f(x))/dx}}}
So the numerator is {{{f(t)-f(x)=7/(t-4)-7/(x-4)}}}
and the denominator is {{{dx}}}.