First you must note that the sum of two supplementary angles is 180°
Also, since the sum of angle on a straight line is 180°, the angles that are supplementary must lie on the same straight line.
Based on the diagram, the pair of angles that lies on the same straight line are ∠4 and ∠3, ∠4 and ∠5 and ∠3 and ∠6. If that is the case, hence this angles are supplementary.
the graph is decrasing, so the variable rate is negative (-2/3)
the rest was quite easy, you just take a look at where the graph cuts the y-axis and determine the rest from the rate of going up or down (increase/decrease)