!x! is the absolute value of x, that's what that means..in simple terms, regardless of whether x is positive or negative the value of x will be positive.
Linear equations must be x to the power of 1, so the answer is B, but more knowledge: C. is a quadratic, i.e. to the power of 2 D. is an exponential i.e. to the power of n, (in this case n equals x). .
