Posted by josh on Thursday, November 11, 2010 at 7:52pm.
recall the properties of parallelogram,,
one of the properties of a parallelogram is that the opposite angles are congruent/equal,, since you are given angles A and C, and they are opposite, equate them to get x:
4x - 57 = x + 3
combining similar terms,
4x - x = 57 + 3
*when transposing terms to other side of equation, the sign becomes the opposite,, that's why x (from right side) when transposed to left side becomes -x, and -57 becomes 57 after transposing,, simplifying this,
3x = 60
x = 20
hope this helps. :)