3. On the board, there was a formula for a fractional-linear function of the form $f(x)=\frac{a x+b}{c x+2}$, where $a, b$, and $c$ are some real numbers (not necessarily distinct). Vasya swapped the numbers $b$ and $c$, and it turned out that the formula now written on the board is the inverse of the function $f(x)$. Find the number $a$. If there are multiple possible answers, write them in any order separated by a semicolon.
Answer: $2 ;-2|2,-2|-2 ; 2$

## Examples of writing answers:

$1 / 4$

$-0.25 ; 10$

## Grade 9

#