First get everything to have the same base of 5
25^11 - 5^19
(5^2)^11 - 5^19
5^(2*11) - 5^19
5^22 - 5^19
Now factor out the GCF 5^19 to get
5^22 - 5^19
5^(19+3) - 5^(19+0)
5^19*5^3 - 5^19*5^0
5^19(5^3 - 5^0)
5^19(125 - 1)
5^19*(124)
At this point, we factor the 124 into 31*4 to end up with this full factorization: 5^19*31*4
Therefore, 25^11 - 5^19 is equivalent to 5^19*31*4
Since 31 is a factor of the original expression, this means the original expression is divisible by 31.