If the two equations $x^2-Kx-21=0$ and $x^2-3Kx+35=0$ have a common root, and K > 0 then the value of K is

Two equations $a_1{x}^2+b_{1}x+c_1=0,a_2{x}^2+b_{2}x+c_2=0,where{a}_1{b}_2-a_2{b}_1\ne 0$ have common root, if 

${\left({\text{c}}_{\text{1}}{a}_2-c_2{a}_1\right)}^2=\left(a_1{b}_2-a_2{b}_1\right)\left(b_1{c}_2-b_2{c}_1\right)$

$\Rightarrow {\left(-21-35\right)}^2=\left(-3K+K\right)\left(-35K-63K\right)$

  $\Rightarrow K^2=16$

$\Rightarrow K\text{= 4}$