explain how it is possible to determine whether the graphs of 5x-3y=8 and 5x-3y=8 are parallel with out doing calculations ​

The diagram I attached is this triangle based on the information you gave. From the information, we know a few things that can help us solve the problem.
(You should know that a triangle has 180°, as well.)

First: We know that m∠A = 20°. We know that m∠B = m∠C. And finally, we know that m∠A + m∠B + m∠C = 180. Since m∠B = m∠C, we can substitute one for the other. Let's say that m∠B = x, just for clarity's sake. We end up with:
20 + 2x = 180
2x = 160
x = 80°
This means that m∠B and m∠C = 80°.

To solve for AC, we can use the Law of Sines (which is ). Substitute 20 for A, 4 for a, 80 for B, and AC for b.

Cross-multiply:

Divide both sides by .

And finally, solve: