You would add 25 to the expression a perfect square trinomial has terms a^2+2ab+b^2 (variables a and b stand for the #s in the original expression not the 'x') a^2=1 2ab=10 Solve for b!! a=1 since 1×1=1 b=5 since 2 (1)b=10 2b=10 b=5 now solve for the last term b^2 5×5=25 25 is what needs to be added into the expression.. Hope this helps