First observe the triangle, look at how the two sides are congruent. With that you can realize that the bottom right angle (3x+7) is congruent I the bottom left, meaning the bottom left angle is also 3x+7.
Next you would want to add 5x+13 to 3x+7 which will equal 180 because the two angles are on the same line and a line is 180 degrees. The equation would look like this after adding all like terms together: 8x+20=180. Next subtract 20 from both sides which will then look like 8x=160. Next isolate x by dividing by 8 on both sides to get x=20
When you add a zero to the problem, it shows you are now multiplying in the double-digits. Let's say you are multiplying 12x12. First you cover up the one then multiply them. Then cover up the two and you have 10. When you multiply them, it ends in a zero. Don't think of it as adding a zero but adding to products.
