if 3 x + y = 81 and 81 x - y = 3, then what is the value of x?

Given, 3^{x + y} = 81
⇒ 3^{x + y} = 3⁴
⇒ x + y = 4 ...(i)
and 81^{x - y} = 3 or (3⁴)^{x - y} = 3
⇒ x - y = 1/4
On solving the Eqs. (i) and (ii), we get
2x = 17/4 ⇒ x = 17/8