You could type in -0x+Y=-3 or 0x-Y=3 if you want x to be positive
The line of the graph has a slope of 0 so if you put it in y intercept form it would be
y=0x-3 so if you subtract x from both sides you would get -0x+Y=-3 and that would put the equation into standard form. I’m not 100% sure if it’s correct so I’m hoping it’s correct