x is an input to the function parameterized_fitness(). Its values are defined by the code that calls this function, not within this function itself.
Also, you need not re-define a and b since these are passed in as well. The usage of this function would look to be something like:
x = [-0.5 0.5];
a = 100;
b = 1;
y = parameterized_fitness(x, a, b);