49s2-64t2

Rewrite 49s2 as (7s)2.

(7s)2-64t2

Rewrite 64t2 as (8t)2.

(7s)2-(8t)2

Since both terms are perfect squares, factor using the difference of squares formula, a2-b2=(a+b)(a-b) where a=7s and b=8t.

(7s+8t)(7s-(8t))

Multiply 8 by -1.

(7s+8t)(7s-8t)

Factor 49s^2-64t^2