You have the following equation:
x - 3 = x²
In order to determine if the given equation is linear in x, you consider that a linerar equation is an equation in which the maximum exponet of the variable is 1. In this case you have that the higher exponent of the variable, which is x, is 2. 2 is the higher exponent, Becuase of that, the given equation is not linear, instead of that, the equation is quadratic.