Explanation:
this creates a quadrilateral.
that means the sum of all internal angles is 360°.
each internal angle is the supplementary angle of the specified expressions (that means they are together 180°).
so, the internal angle of 6x is 180 - 6x.
the internal angle of 5x is 180 - 5x.
and so on.
as mentioned, the sum of these 4 internal angles is 360.
therefore,
180 - 6x + 180 - 5x + 180 - (x + 16) + 180 - (3x - 1) = 360
720 - 11x - (x + 16) - (3x - 1) = 360
720 - 11x - x - 16 - 3x + 1 = 360
705 - 15x = 360
345 = 15x
x = 345/15 = 23