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