is a standard quadratic equation which we can solve 
through factorization.
Rewrite the equation, splitting 7x as 8x - x



So the 2 roots are:
 or 
 or 
Hope you got it:)
See the graph below:

 
Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation  (in our case ) has the following solutons:
  
  
  
  For these solutions to exist, the discriminant  should not be a negative number.
  
  First, we need to compute the discriminant : .
  
  Discriminant d=81 is greater than zero. That means that there are two solutions: .
  
    
    
    
    Quadratic expression  can be factored:
  
  Again, the answer is: 0.5, -4.
Here's your graph: