Ad¯¯¯¯¯ , bd¯¯¯¯¯ , and cd¯¯¯¯¯ are angle bisectors of the sides of △abc . be=12 m and bd=20 m

Since triangle BDE is right, we can check it for special rules: BD is hypotenuse = 20, and BE = 12, both divisible by 4: get 5 and 3... so it's a 3-4-5 special right triangle, the 4×4 = 16 m for side DE.
Since AD bisects angle A, I think then side DG should be congruent with DG. So then
DG = 16 m

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table