Click and type
x = 4
A laptop has no pen, so typing has to work
Writing with a trackpad is slow, and most worksheet tools treat typed text as an afterthought. On a laptop, typing is how the work gets done, so the keyboard has to be enough: click where the answer goes and type it. Text blocks can carry formatting, and exponents and subscripts can be typed without reaching for a menu.
For anything harder, add a typeset expression, a graph or a table right on the page. If you do have a touch screen or a stylus, those draw too.