\(\text{a) }-x^2+10x-25\\ =-\left(x^2-10x+25\right)\\ =-\left(x-5\right)^2\)
\(\text{b) }-x^2-6x-9\\ =-\left(x^2+6x+9\right)\\ =-\left(x+3\right)^2\)
\(\text{c) }-4x^2+20xy-25y^2\\ =-\left(4x^2-20xy+25y^2\right)\\ =-\left(2x-5y\right)^2\)
\(d\text{) }!!!\)