中一數學題,,,help

(a+3):(a+5):b=3:4:7
that means
(a+3):(a+5)=3:4
(a+3)/(a+5)=3/4
4(a+3)=3(a+5)
4a+12=3a+15
a=3
And then
(a+5):b=4:7
(3+5):b=4:7
8/b=4/7
7x8=4b
4b=56
b=14
if a:b:c=x:y:z
a/x=b/y
b/c=x/z
a:b=a/b

(a+3)÷3 = (a+5)÷4

4a+12 = 3a + 15

a = 3

a+3　 3
------=----
a+5　 4

4(a+3)=3(a+5)
4a+12=3a+15
a=3

a+5　 4
------=----
　b 　 7


7(3+5)=4b
b=14