If your equation is A = 3(B-25)/7, then we can solve for B by isolating B on one side. We can do this by the following...
First, multiply both sides by 7
7A = 3(B-25)
Next, divide both sides by 3.
7A/3 = B - 25
Finally, add 25 to both sides.
7A/3 + 25 = B
Therefore, B = 25 + 7A/3