As Babak points out, we assume the domain of our function is restricted to the interval $(-10, 13)$: 


When $f(x) =4,\;$ $f(f(4)) = 15:\;$ Find the x-values that correspond to all the points where $f(x) = 4$: you should find four such $x$-values: $x = -8, 1, 7.5, 10$.
When $f(x) = 12$, note that $f(f(12)) = 15:$Find the x-values that correspond to where $f(x) = 12$. Again, there should be three or four such values, depending on whether you include the value at the far, far left of the graph: (approximations by "eying up" $x =? = -10, x = 3.5, 5.25, 11.5$


The figure below shows the graph of a function $f(x)$. How many solutions does the equation $f(f(x)) = 15$ have?
Is it possible to achieve citations that are superscript and don't have brackets using the revtex4-1 document class?

An example of how they should look: 


  Afirmation3,4

