Suppose I have the equation $x-a=0$. So, $x-a=0\Rightarrow x=a$. The unique solution is (or root) $a$ (by definition a solution is the value for the unknown that make the equation true, $a-a=0$ ). Since $x=a$ shouldn't x(the unknown) considered a solution?
Sorry if the question isn't clear but i have this stupid doubt.
In the equation $x = a$, the $x$ is a variable rather than a number. The way you want to think about it is: What can I put in the brackets so this equation is true? $$[\ ] = a$$
The "$x$" doesn't exist on its own as a number, it's like having brackets that say "put some numbers in me!"
If you put $a$ in the equation, then you get $a = a$, which is true. If you put somthing which isn't $a$ in the equation, it's false. However, $x$ is not a solution because it is just a placeholder that represents "where you plug numbers into your equation".
However, a bit of care needs to be taken: It is correct to say "$x$ is the solution if $x = a$". In this case $x$ is no longer a variable, but it's literally the value of $a$.
EDIT
The part above was a bit ambiguous, I've tried to clarify it a bit. It depends on context. The "equation" $x = a$ is equivalent to the "function" $f(x) = x-a$, where a "root" of the equation is a "zero" of the function. However, if you have some number $a$ and define $x$ to be equal to $a$, usually denoted $x := a$ then it's no longer a function or an equation - $x$ is no longer a variable, it's the same object as $a$. Thus $x = a$ can have different meanings - either $x$ is a variable and $x=a$ is an equation, or $x$ is a constant, and $x=a$ is a definition.