The subject of the formula means that you have to get some value equal to everything else. I'll do the first three, and you can go from there!
Our first step would be to separate the coefficient of the value from everything else (the coefficient is what the value is multiplied by). Therefore, in 2x-4=y, we have to add 4 to both sides to separate 2x, resulting in 2x=4+y. Next, we have to divide by the coefficient to separate x (in this case, 2), resulting in x=(4+y)/2
For the next one, we repeat the process - add 6 to both sides since it's subtracted by 6 (conversely, if we were to be adding 6 to 8b originally we would subtract 6 from both sides) to get 8b=6+a. Next, we divide both sides by 8 to get b=(6+a)/8.
For the third one, we add 1 to both sides to get 4d=c+1 and then divide both sides by 4 to get d=(c+1)/4