the sum of all interior angles in a polygon is
180(n - 2), where n = "number of sides in the polygon"
now, this polygon has 5 sides, so is a PENTAgon, so the sum of all its interior angles is 180( 5 - 2), or 540.
we know 4 of them, so the last one, the one next to A, has to be 540 - 104 - 118 - 96 - 115, namely is 107.
now, notice, that angle is next to A, is really sitting on the same exact line angle A is sitting on, so that angle is a linear angle with A.
what the dickens does that mean? well, linear angles make up 180°, therefore,
107 + A = 180
A = 180 - 107
