$\frac{ \left( 5-k \right) \times \left( 5+k \right) }{ {k}^{2}-10k+25 }$
Use ${a}^{2}-2ab+{b}^{2}={\left( a-b \right)}^{2}$ to factor the expression$\frac{ \left( 5-k \right) \times \left( 5+k \right) }{ {\left( k-5 \right)}^{2} }$
Factor out the negative sign from the expression and reorder the terms$\frac{ -\left( k-5 \right) \times \left( 5+k \right) }{ {\left( k-5 \right)}^{2} }$
Cancel out the common factor $k-5$$\frac{ -\left( 5+k \right) }{ k-5 }$
Use $\frac{ -a }{ b }=\frac{ a }{ -b }=-\frac{ a }{ b }$ to rewrite the fraction$-\frac{ 5+k }{ k-5 }$