This expression can be simplified as follows:

(25m)^2 - (3m - 1)^2

Expanding the squares:

(25m 25m) - ((3m - 1) (3m - 1))
625m^2 - (9m^2 - 6m + 1)
625m^2 - 9m^2 + 6m - 1

Combining like terms:

625m^2 - 9m^2 + 6m - 1
(625 - 9)m^2 + 6m - 1
616m^2 + 6m - 1

Therefore, the simplified expression is 616m^2 + 6m - 1.