is it compulsory to write n*2 as 2n

No, as both of them are of the same value,
n*2 or n x 2 = 2n or n2

No it is not nesecerry to write it because both same value

Hello Rohan, of course you could write as n2 for 2n. But some times we n₂ with suffix. So n2 could be mistaken as that one. So we also prefer writing numeral first and alphabet following it. 3 apples, 6 roses, 15 horses, same way 46 a , 54 a b^2 ,
233 a^3 b^3. Hope you have felt the convenience of writing so