What is the equation of the horizontal line that has a y intercept of - 4?

Since the line is horizontal (flat) the gradient, m, is zero (0). c is the y-intercept and in this case - 4.

Equation of a line: y = mx + c

since m = 0 then,

y = c = - 4

therefore y = - 4 ... is the equation.

The answer is y=-4 because you don't have a x - coordinate