x is equal to 27.
In order to find the value of x, you have to use the average formula and solve for the missing variable. The mean formula is all of the numbers added together divided by the number of inputs.
(inputs added)/(number of inputs) = Average
(2 + 7 + x)/3 = 12 ----> Multiply by 3
9 + x = 36 ----> Subtract 9
x = 27