If Joel has 2x the amount of cd's as M then a equation would be J = 2M. The equation for Blake is B = (J-7)/3. Since we know J is equal to 2M we can sub that in to make it. B = (2M - 7)/ 3. We also know that Blake has 25 cds so we can sub that in to make it 25 = (2M - 7)/ 3. Then we can begin to solve. Multiply both sides by 3 to make it, 75 = 2M - 7. Then add 7 to both sides to make 82 = 2M then divide by 2 to get M = 41.