Solve the equation by completing the square 5x(x+6)=-50

First lets distribute the 5x 
5x^2 + 30x = -50

Lets divide every term by 5 
x^2 + 6x = -10 

To complete the square we have to half the b value, which in this case is 6. Then square it. 
Half of 6 is 3, 3 squared is 9 

Add that to both sides of the equation 
x^2 + 6x + 9 = -1 

Find the binomial squared
(x+3)^2 (If you're wondering how i got that please comment) 

(x+3)^2 = -1 

Take the square root of the equation of both sides
(x+3) = +/- i
x = -3 +/- i
x = -3 - i
and 
x = -3 + i