finding the equation

You've presented a single point. If finding a horizontal line, only the y-value is relevant. In the case, the y-value is 3.The only line that will pass through a point (#,3) is y=3. If the point was (1,3), (0,3), or (-364,3) the answer remains y=3.

To follow up on this question, ask yourself why only the y-value matters. Try plotting y=-1, y=1, y=4. Then try plotting x=-1, x=1, and x=4. It should become apparent why only the y-value matters with a horizontal line and only the x-value matters with a vertical line.