We plot the point (-3,2) which the line is to go through:

 

Since the line's slope is 0 it is a horizontal line.  We'll draw it:



We see that its y-intercept is (0,2) so b = 2.

And we are given that the slope m = 0, so we substitute in

y = mx + b

and get

y = 0x + 2

But we usually don't write "0x" but it's still understood:

y = 2

That's the equation.  Horizontal lines' equations do not contain
an x-term because it is just multiplied by 0 and usually not written,
but it's understood that the reason it's not written is because the
slope is 0.  So think of it as y = 0x + 2 but just write y = 2.

Edwin