I assume you want the sons to have equal amounts of money, then the solution is easier than you might think.
Let x = the value of the father's will, then
The first son receives dollars.
The second son receives dollars.
As these are equal, equate and solve for x:
Now it's easy to find the number of sons. Since now we know the first son receives 6 dollars, and all the sons receive the same amount, the man has sons.
I can't believe how long I spend looking at functions of functions and trying to turn everything into massive overcomplicated expressions, all the while overlooking this easy solution.