x^2 + y^2 = 13 is a circle centered at (0,0) with a radius of sqrt(13). It's not in slope intercept form because it's not a line.
y = 2 is a horizontal line at y = 2. It can be thought of as y = 0x + 2. Both y = 2 and y = 0x + 2 are in slope intercept form.