I think the missing number should be
12.87
Try it with some random number, like a=3.
Is "3+0 = 0+3 = 0" true? No.
If you were doing multiplication, then it would be true, but not with addition.
The answer is 35
Explanation: because 42 divided by 6 is 7 and 7 times 5 is 35
AB = 9 cm
BC = 6cm
CD = 7 cm
AE = 6 cm
3BC = AB
3ED = AE
AB = AE
BC ED
⁹/₃ = ⁶/ₓ
3 · 6 = 9 · x
18 = 9x
9 9
2 = x
ED = 2 cm