If the square of twice a number is 81, what is the number? I tried doing 2x^2=81 but that did not come out to the answer 4.5,-4.5

If the number is n then

square of 2 times a number

square of 2n

(2n) ²

that was your mistake, because (2n) ²=4n², not 2n²

the result is 81

(2n) ²=81

square root both sides

remember positive and negative roots

2n=+/-9

divide by 2

n=+/-4.5

n=4.5 or - 4.5