If we are to assume that a is an exponent on the x, then a would have to be 1, because x^1 is the same thing as just plain old x. If we put an exponent other than a 1 on that x, it is no longer linear. If we put an exponent of 2, the function would be a quadratic. If we put an exponent of 3, the function would be a cubic. If we put an exponent of 4, the function would be a quartic. Linear is just a plain line in the form y = mx + b.
