Solve for I on P = I x E.
Since, E is being multiplied on right side, we divide both sides by E.
`(P/E) = (I x E)/E`
Cancel common factor on top and bottom of right side.
`P/E = I` or `I = P/E`
Replace the I on E = I x R by what we got.
`E = (P/E) x R`
Multiply both sides by E.
`E x E = E x (P/E)x R`
`E^(2) = P x R`
Take the square root of both sides.
`E = sqrt(P x R)`
Eliminate I from both equation (only I prsent in both equation)
from (i) and (ii) ,we have
which is required formula.