To find \( x \) such that \( f(x) = 12 \) where \( f(k) = k^2 - 4 \), we can set up the equation:
\[ k^2 - 4 = 12 \]
Now, add 4 to both sides:
\[ k^2 = 12 + 4 \] \[ k^2 = 16 \]
Next, take the square root of both sides:
\[ k = \pm 4 \]
Thus, the solutions for \( x \) are:
\[ x = 4 \quad \text{or} \quad x = -4 \]
So the values of \( x \) that satisfy \( f(x) = 12 \) are \( x = 4 \) and \( x = -4 \).