Question

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

Answer 1

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