If his base pay is $300, then subtract it from $800, you get $500. Then divide $500 by how much Alex earns per table, $50, then you get 10. Alex needs to make 10 tables per week.
It shouldn't add up to 180 degrees, you're looking for the length of the sides not the degree of the angle. Because it's an equilateral triangle, all sides are the same, so you can just set them equal to each other. D=5
The length of FG would be BC. This is because FG and BC corresponds on the shape letter sequence. This would mean that EF would be AB, that GH would be CD, and that HE would be DA.