Solution
For this case we can do the following:
[tex]x^2+20x+(\frac{20}{2})^2=-87+(\frac{20}{2})^2[/tex]Then when we simplify we got:
[tex]x^2+20x+100=-87+100[/tex]If we simplify we got:
[tex](x+10)^2=13[/tex]Taking square root we got:
[tex]x+10=\pm\sqrt[]{13}[/tex]Then the two solutions are:
[tex]x_1=-\sqrt[]{13}-10,x_2=\sqrt[]{13}-10[/tex]