The power is not zero, the power is one only but its not written as its understood....... For eg. 4x.... Its power is 1 and hence is linear. An equation can't have 0 power .. So its one (1). How can u multiply a number zero times.. Like if it was 4x^2 , This means 4x is to be multiplied two times and if its only 4x then u have to not multiply it further with itself.