Looking at the x part. If you want to get it into the form (ax+b)2 expanding it out might make it clearer. (ax+b)2=a2x2+2abx+b2
since the x2 part is 25x2 you know that a2=25, a=5
The x part is -300. You know that 2ab=-300. and a=5 so b=-30
Lastly you need to balance the square by subtracting a constant.
For this to be true c must be equal to -900