Change in subject Formula

 $I=PRT$  
this means that we want to get P on one side of the equation, on it's own. 
right now  $I$   is the subject of the formula .

 $I=P\times R\times T$  we know that to get rid of value that is being multiplied, we simply divide by that same value. so here we need to divide by  $R\times T$  
bearing in mind that whatever you do to one side of the equation you do to the other.

 $\frac{I}{R\times T}=\frac{\left(P\times R\times T\right)}{R\times T}$  


so we cancel the R and T on the right hand side of the equation (side with the P) which leaves us with  $\frac{I}{R\times T}=P$  or we can rearrange   $P=\frac{I}{RT}$  

make  $l$  the subject. 

 $V=lbh$