subtract and simplify by collecting like radical terms if possible

4�ã8-2�ã2

your symbols came out strange on my computer

did you mean 4√8 - 2√2 ??
if so, then ...

= 4√4√2 - 2√2
= 8√2 - 2√2
= 6√2

or

4√8 - √8
= 3√8
= 3(2√2) = 6√2