Check every pair to see if it's a solution.
Here's how you can check if a pair is a solution:
Plug it into the equation.
Multiply the x coordinate by 6, and the y by 3. Add these up. If you get 15, then it's a solution.
Here are a hundred solutions:
(-50,105) (-49,103) (-48,101) (-47,99) (-46,97) (-45,95) (-44,93) (-43,91) (-42,89) (-41,87) (-40,85) (-39,83) (-38,81) (-37,79) (-36,77) (-35,75) (-34,73) (-33,71) (-32,69) (-31,67) (-30,65) (-29,63) (-28,61) (-27,59) (-26,57) (-25,55) (-24,53) (-23,51) (-22,49) (-21,47) (-20,45) (-19,43) (-18,41) (-17,39) (-16,37) (-15,35) (-14,33) (-13,31) (-12,29) (-11,27) (-10,25) (-9,23) (-8,21) (-7,19) (-6,17) (-5,15) (-4,13) (-3,11) (-2,9) (-1,7) (0,5) (1,3) (2,1) (3,-1) (4,-3) (5,-5) (6,-7) (7,-9) (8,-11) (9,-13) (10,-15) (11,-17) (12,-19) (13,-21) (14,-23) (15,-25) (16,-27) (17,-29) (18,-31) (19,-33) (20,-35) (21,-37) (22,-39) (23,-41) (24,-43) (25,-45) (26,-47) (27,-49) (28,-51) (29,-53) (30,-55) (31,-57) (32,-59) (33,-61) (34,-63) (35,-65) (36,-67) (37,-69) (38,-71) (39,-73) (40,-75) (41,-77) (42,-79) (43,-81) (44,-83) (45,-85) (46,-87) (47,-89) (48,-91) (49,-93)