Enter the sum (ensuring a space between the factors) and press the Enter key.

Context Panel: Expand

Context Panel: Collect≻$x$

Sum of the coefficients of $x^3$ and $x^4$ by control-drag (or copy/paste). Press the Enter key.

Assign the sum to $q_1$.

$q_1≔{\left(2 x-a\right)}^4\cdot {\left(x\+a\right)}^2\+{\left(3 x-1\right)}^5$

Apply the expand command.

$q_2≔\mathrm{expand}\left(q_1\right)$

Apply the collect command.

$q_3≔\mathrm{collect}\left(q_2\,x\right)$

Use the coeff command.

$\mathrm{coeff}\left(q_3\,x\,3\right)\+\mathrm{coeff}\left(q_3\,x\,4\right)$