Solving for y just means rearranging the equation until y sits by itself on one side, with everything else on the other. The equation itself does not change, only the way it is written.
B = 705y/h2
Right now y is tied up in two ways. It is multiplied by 705, and the whole thing is divided by h2. So we undo those two, one at a time, and it is easiest to clear the fraction first.
Multiply both sides by h2. On the right side that cancels with the h2 underneath:
Bh2 = 705y
One thing worth noticing here, because it is where people usually hesitate: h2 moves as a single block. You do not handle the h and the exponent as separate steps. Whatever h2 works out to be, it is one quantity, and it multiplies or divides all at once.
Now y is only being multiplied by 705, so divide both sides by 705:
Bh2/705 = y
And turned around the way we normally write it:
y = Bh2/705
A good habit after any rearrangement is to check it by putting your new version back into the original equation. Replacing y with Bh2/705 makes the right side 705(Bh2/705)/h2. The 705 on top cancels the 705 on the bottom, the h2 cancels the h2, and you are left with just B, which is exactly what the original equation said. That check costs about ten seconds and catches almost every algebra slip.
If this formula looks familiar, it is the BMI one, where B is the BMI, y is weight in pounds and h is height in inches. That is also why rearranging it is useful rather than just an exercise. The original form answers what BMI goes with a given weight, and the version we just found answers the reverse question, what weight corresponds to a target BMI at a given height.
So, our final answer is y = Bh2/705.
