A horizontal line passing through the point (5,2) would be y = 2.
Each y-coordinate has a value of 2, no matter the x-coordinate.
Slope is rise/run. In the line y = 2, there is no rise, only run.
Standard form is Ax + By = C. With the equation being y = 2, it is already in standard form.
If you want to be picky, you could write it as 0x + y = 2.
y = 2