Solution
For this case we can complete the square on this way:
[tex]x^2+11x+(\frac{11}{2})^2+24-(\frac{11}{2})^2[/tex]and if we simplify we got:
[tex](x+\frac{11}{2})^2+24-\frac{121}{4}=(x+\frac{11}{2})^2-\frac{25}{4}[/tex]We can use this property:
[tex](a^2+2ab+b^2)=(a+b)^2[/tex]For this case:
[tex]a=x,b=\frac{11}{2}[/tex]