Let's deal with the two inequality, and then put them together. To see where an inequality holds, you need to see where the equality holds first. Then, if you have an inequality like #\le#, you'll take all the points "before" the equality holds, otherwise with an inequality involving #\ge# you'll take all the points "after" the equality holds, and now I'll explain what I mean with "before" and "after".

Let's start from #y \le -1#. The associated equality is #y=-1#. This is a horizontal line. So the set #{y | y\le -1}# is the part of the plan below that line (since the #y#-axis is vertical, "before" means below.

As for #x\ge 3#, we have that #x=3# is a vertical line. So, the set #{x | x\ge 3}# is the part of the plan to the right of that vertical line (since the #x#-axis is horizontal, "after" means "right"

So, to solve the system, impose the two condition at the same time: you'll take all the points below the line #y=-1#, and all the points to the right of the line #x=3#