Control-drag (or type) the expression.

Context Panel: Assign to a Name

Type the equation $g\left(x\right)\=\dots$

Context Panel: Assign Function

Type the notation for the desired value.

Context Panel: Evaluate and Display Inline

The syntax for defining a function is shown to the right.

$g≔x\to x^2 \cos \left(2 x\right)$

Assign the given expression to q.

$q≔x^2 \cos \left(2 x\right)$

Use the unapply command.

$g≔\mathrm{unapply}\left(q\,x\right)$

Type the notation for the desired value and press the Enter key.

$g\left(2.7\right)$