Answer: The required value of x when y = 4 is 27.
Step-by-step explanation: We are given to find the value of x in the following equation when y = 4 :

To find the value of x, first we should write equation (i) as y as the dependent variable.
From equation (i), we have

Now, substituting y = 4 in equation (ii), we get

Thus, the required value of x when y = 4 is 27.