Is there a symbol, or abbreviation for coefficient of x

Question

Doing some binomial expansions with algebra where I need to equate different coefficients together but don't know what to write:

[Coefficient of $x^3$] = _

$k=+1.5$

Answer 1

For an expression $f(x)$, there is the notation $$[x^3](f(x))$$ See MathWorld. This notation is verbose but can be useful for expressions in several variables.

For an expression in a single variable, I'd go for something simpler, like $$[f(x)]_3$$ but be explicit when you introduce or use non-standard notation.

Answer 2

Why not just something like $a_{xn}$? Where x is the variable name and n the exponent. This seems to me more easy that the one given by @lhf.

You need to define the notation explicitly anyway and seems that no "standard" notation exist for this.