Answer:
y = 1
Explanation:
The point-slope form of an equation of a line:

m - slope
(x₁, y₁) - point on a line
We have m = 0, (3, 1) → x₁ = 3, y₁ = 1.
Substitute:

add 1 to both sides

Other method.
If the slope is equal 0, then it's a horizontal line.
The equation of a horizontal line: y = a.
We have the point on a line (3, 1) → x = 3, y = 1.
Therefore the equation is y = 1.