I know it’s simple but my brain isn’t working, someone please explain this.

X ≈ 9.33 units

Explanation:
As triangle DOG is similar to triangle CAT, the ratios of the sides are the same

AT/OG = CA/DO
x/4 = 7/3
x = 28/3
x ≈ 9.33

Factors of 12
=1, 2, 3, 4, 6, 12
Factors of 12 that add up to numbers
Factors of 12 that add up to 28 =1 + 2 + 3 + 4 + 6 + 12

Factors of 12 that add up to 3 = 1 + 2

Factors of 12 that add up to 6 = 1 + 2 + 3

Factors of 12 that add up to 10 = 1 + 2 + 3 + 4

Factor of 12 in pairs
1 x 12, 2 x 6, 3 x 4, 4 x 3, 6 x 2, 12 x 1
1 and 12 are a factor pair of 12 since 1 x 12= 12

2 and 6 are a factor pair of 12 since 2 x 6= 12

3 and 4 are a factor pair of 12 since 3 x 4= 12

4 and 3 are a factor pair of 12 since 4 x 3= 12

6 and 2 are a factor pair of 12 since 6 x 2= 12

12 and 1 are a factor pair of 12 since 12 x 1= 12