A horizontal line is achieved when the gradient of the line is zero. Hence for this to happen, ig you need to ensure that 'e' to the power of x nought is equal to one or an integer multiple of that (so you could equate it that way). In addition, you could also make the converse true, wherein y nought is equal to zero and x nought is not!!! This will give y = 0, which is effectively a horizontal line....