To factor a number out of an expression means to divide each term of the expression by that number.
So, if you want to factor [tex] a [/tex] from the expression [tex] x+y+z [/tex], you'll write it as
[tex] a\left(\dfrac{x}{a}+\dfrac{y}{a}+\dfrac{z}{a}\right) [/tex]
So, in your case, when you divide both terms by -5 you get
[tex] -10a \div (-5) = 2a,\quad -25\div (-5) = 5 [/tex]
So, the expression becomes
[tex] -10a-25 = -5(2a+5) [/tex]
