(1)∵点M到y轴的距离为2,∴4a-8=2或4a-8=-2,解得a= 5 2 或a= 3 2 ,当a= 5 2 时,a+3= 5 2 +3= 11 2 ,当a= 3 2 时,a+3= 3 2 +3= 9 2 ,所以,点M的坐标为(2, 11 2 )或(-2, 9 2 );(2)∵点N(3,-6),直线MN∥x轴,∴a+3=-6,解得a=-9,∴4a-8=4×(-9)-8=-36-8=-44,∴点M(-44,-6).