Let the numbers be a and b
Given that the sum is 17 then
a + b = 17
also given that one number is 3 less than the other
a = b + 3
substituting
b + 3 + b = 17
2b + 3 = 17
subtract 3 from both sides
2b = 17 - 3
2 b = 14
divide both sides by 2
b = 14/2
= 7
The lesser of the two is 7