In a series circuit U = U₁ + U₂ + ... + U_n where U = applied voltage from a source and U₁, U₂, ... U_n = voltage over each resistor.

If a $\displaystyle {21}$ V source over three resistors has volts over two resistors of $\displaystyle {9}$V and $\displaystyle {8}$V, what is the voltage over the third resistor?

U₃ = V