Question 229367
25a^2+16-40a Place the terms in a standard quadratic formula format:
[ax^2+bx+c]
25a^2-40a+16 If the a & the c terms are perfect squares & the b term is 
2*sqrt(a+c), as in this case, then it is the square of the sum or difference of the 2 square root terms (5 & + or -4, depending on the + or - value of the middle term.)   
(5a-4)^2 ans.
Hope this helps. 
You'll get it with a little more practice.