Introduce new variable  u = .  


Then our equation becomes


    u^2 -3u - 10 = 0.


Factor left side


    (u-5)*(u+2) = 0.


The solutions are  u= 5  and  u= -2.


a)  If u = 5,  then   = 5,  hence x = 25.


b)  if u = -2, we reject  the possibility for   to be negative, as your teachers and your textbooks teach you at the school.


ANSWER.  x= 25.