This is what we call a conjugate binomial which is two binomials with similar terms but with one of those terms with different signs, this shows a result of the first term squared minus the second term squared.
The formula for conjugate binomials are:
(a+b)(a-b)=
In this case the multiplication of
(7t³ + )(7t³ - )= 49 + 14 - 14 -
After substracting the second and the third term you are left with: