Rewrite the equation as: b = 3(a + 2)/(a + 5)
Let z = a+5 z is also an integer
Then b = 3(z-3)/z or b = 3 - 9/z
For this to result in an integer, we must have 9/z as an integer. This means the following values are the only ones allowed for z:
z = -9, -3, -1, 1, 3, 9 which means a = -14, -8, -6, -4, -2, 4 with corresponding values for b of: b = 4, 6, 12, -6, 0, 2
I'll leave you to order them in pairs.