SOLUTION: If f(3x/x-4) = x² + x + 1, what is the value of f(5)?

To solve the problem, find the value of x such that

     = 5.


It is easy: first, cross multiply to get

    3x = 5(x-4),


then simplify step by step

    3x = 5x - 20

    20 = 5x - 3x

    20 = 2x  --->  x = 20/2 = 10.


Now substitute x= 10 into the given formula. Since   = 5 at  x= 10,  you will get then

    f(5) =  = 100 + 10 + 1 = 111.


ANSWER.  Under imposed conditions,  f(5) = 111.