it is (c+d)
you can basically use FOIL (first,outside,inside,last) to solve this.
you know the first term of the factor has to be C because of the c^2
the second then has to be D because you haven't seen D in the factor yet and it is in the equation.
then you know from the equation that the terms with D are not negative so D isn't negative so you have form the information above (c+d)