Question

How to tell if 2 lines are parallel formula?

Answer 1

If the Co-Interior Angles is equal to 180 or Corresponding Angles or Alternate Interior Angles are equal the lines are parallel

Answer 2

is that u are asking from chapter linear  equations????

if yes then here is the formula-

a1/a2 = b1/b2 is not equal to c1/c2

hope it helps