a^2 - b^2 =100
(a + b) ( a - b) = 100
Factors of 100 = 1 , 2 , 4 , 5 , 10 , 20 , 25 , 50 , 100
a + b = 100
a - b = 1
2a = 101
a not an integer
a + b = 50
a - b = 2
2a = 52
a = 26
b = 24
a + b = 25
a - b = 4
2a = 29
a not an integer
a + b = 20
a - b = 5
2a = 25
a not an integer
a + b = 10
a - b = 10
2a = 20
a = 10
b = 0 (not a positive integer)
So
(a , b) = (26,24 )