Determine when value will be greater than x I want every time to add '$(b-x)$' to the value of $a$ and i want to know how many times i will repeat this operation to be the value of $a$ greater or equal to $x$. 
$a = b+a - x$
I know that $b > x$ and $ a < x $
Any ideas?

So you have an arithmetic sequence
$t_n = a + n(b - x)$, (Starting at $n = 0$).
where $a < x < b$, and you want to know the first time that $t_n > x$
\begin{align}
   t_n & > x\\
   a + n(b - x) & > x\\
   n(b - x) & > x - a\\
   n & > \dfrac{x - a}{b - x}\\
   n & = \left \lceil \dfrac{x - a}{b - x} \right \rceil
\end{align}
If $\dfrac{x - a}{b - x}$ is not an integer, then
$n = \left \lfloor 1 + \dfrac{x - a}{b - x} \right \rfloor
   = \left \lfloor \dfrac{b - a}{b - x} \right \rfloor$
But if $\dfrac{x - a}{b - x}$ is an integer, then the answer is still 
$\left \lceil \dfrac{x - a}{b - x} \right \rceil$.
