First, recall that the equation of a line is
y = mx+b
where
m = slope
b = y-int
The key to this question is the details :
Horizontal line ---> This tells you that your slope is 0
y-int = 3.5 -----> In the equation above, b=3.5
Therefore,
y = 0x + 3.5
y = 0 + 3.5
y = 3.5