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}=a^{2}x^{2}+2abx+b^{2}

since the x^{2}part is 25x^{2}you know that a^{2}=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