Actually it's 83 not 38. The base of the log is the base of the exponent. log8(xy) = 3 becomes the equation xy = 83 xy = 512 <-- so you had 512, so you must have had 83, not 38. There are many solutions to that. There are infinitely many. Maybe you were to find just the positive integer solutions. If so, the solution (x,y) could be any of these: (1,512) (2,256) (4,128) (8,64) (16,32) (32,16) (64,8) (128,4) (256,2) (512,1) Edwin