In the first column you have the value of x, then to find the mistake you substitute in the equation the given value of x and evaluate to findthe corresponding value of y:
When x is 1:
When x is 2:
When x is 3:
When x is 4:
As you can see the incorrect value of y is when x=1, because when x is 1 y is 1 not 2 as in the given table