If you fill in the x coordinates of both points in the equation, the equation should produce the expected y coordinate. The second equation does that:
(3,7) fill in 3, outcome 7 expected: 2/3*3 + 5 = 2+5 = 7, OK!
(6,9) fill in 6, outcome 9 expected: 2/3*6 + 5 = 12/3 + 5 = 9, OK!
So y=2/3*x+5
