Assuming there is no line under the "less than" sign, this means D + 4 < 7 solves to D < 3, which is what I'm assuming you got. This is found by subtracting 4 from both sides.
Writing D < 3 means D is some number less than 3. A solution is just that. You cannot pick 3 itself because 3 is not less than 3. But you can pick something like 2 since it is less than 3.
D+4 < 7
2+4 < 7 ... replace D with 2
6 < 7 ... true
Or D could be 1
D+4 < 7
1+4 < 7
5 < 7 .... true
and so on.
