[tex]\bf \textit{difference of squares}
\\ \quad \\
(a-b)(a+b) = a^2-b^2\qquad \qquad
a^2-b^2 = (a-b)(a+b)\\\\
-------------------------------\\\\
w^4-21w^2-100\implies (w^2)^2-21w^2-100\implies (w^2-25)(w^2+4)
\\\\\\
(w^2-5^2)(w^2+4)\implies (w-5)(w+5)(w^2+4)[/tex]
