If f(x) = 3x - 7, for what value of x does f(x) = 14?

Set $f(x)=14$: $3x-7=14$. Add $7$ to both sides: $3x=21$. Divide by $3$: $x=7$, choice D. Check: $f(7)=3(7)-7=14$. Choice B ($21$) is the value of $3x$, which stops one step short and does not divide by $3$. Choice A ($5$) gives $f(5)=8$, not $14$, so it does not satisfy the equation. Choice C ($\frac{7}{3}$) divides $7$ by $3$, ignoring the $14$. Always substitute your answer back into the function. Checking by substitution is the quickest way to catch an arithmetic slip.