The threshold frequency of a metal is $f_0$ . When the light of frequency $2f_0$  is incident on the metal plate, the maximum velocity of photoelectrons is $v_1$  . When the frequency of incident radiation is increased to $5f_0$  , the maximum velocity of photoelectrons emitted is $v_2$ .  Then  $v_1+v_2= \sqrt{\frac{2h{f}_0}{m}}\left(x\right),$  where  $x=\_\_\_\_.$