Algebra

 

                                               2x^2 + 3x + 8x - x^2 + 4x + k   

 

Combine like terms                 x² + 15x + k   

 

When the quadratic is in the format like the above,    

to have a double root (also called a repeated root),    

the constant is the square of half the coefficient of x.    

 

Coefficient of x is 15. 

Half of 15 equals 7.5. 

7.5 squared is 56.25.               So, k = 56.25   

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