If the two equations x2−Kx−21=0 and x2−3Kx+35=0  have a common root, and K > 0 then the value of K is

Two equations a1x2+b1x+c1=0,a2x2+b2x+c2=0, where a1b2−a2b1≠0 have common root, if  (c1a2−c2a1)2=(a1b2−a2b1)(b1c2−b2c1) ⇒(−21−35)2=(−3K+K)(−35K−63K)   ⇒K2=16  ⇒K= 4