Solve a linear function How do I solve this homework assignment?

For a linear function $y=f(x)$, $f(-3) = 25$ and $f(3) = 11$.

Determine $f(-20)$.

I know that with the values $f(-3) = 25$ and $f(3) = 11$ I am suppose to derive a formula and the plug in $f(-20)$ but how do I derive a formula from the values given?

Thanks!!
Hint: You don't have to derive the formula. Use the fact that the gradient of a linear function is always constant.

$$\frac{f(-20) - f(3)}{-20 - 3} = \frac{f(3) - f(-3)}{3 - (-3)}$$

Of course, if there is a need, you can derive similarly the formula of $f$. This is, by the way, my favourite method of deriving the equation of a linear function defined by two given points.

$$\frac{f(x) - f(3)}{x - 3} = \frac{f(3) - f(-3)}{3 - (-3)}$$
For a linear function, if you know its value at two points, you can fully determine the function. Do you know that general formula for a straight line in the plane? Write that down, and plug in (-3,25) and (3,11) to solve for the coefficients.