you would equal both expressions to each other.
then you will need to get rid of the square root around 'x+2' by multiplying it to the power of 2.
this will remove the square root. but what you do to one side you must to do the other. therefore, the other side will now be x^4+ 6x^3 +13x^2 +12x +4.
your new expression is
x^4+ 6x^3 +13x^2 +12x +4 =x+2
when you simplify your answer will be
x= -2
