What is the equation of a horizontal line that has a y-intercept of 3.5?
a. y=3.5
b. y=-3.5
c. x=3.5
d. y=x-3.5

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