數學問題(二元一次方程式)(我是中二)(10點!!)

1. 2x+3y=21 ...(1)
x=7y-21+16 ...(2)
From (2),
x=7y-5 ...(3)
Sub (3) into (1),
2(7y-5)+3y=21
14y-10+3y=21
17y=21+10
y=31/17
Sub. y=31/17into (1)
2x+3(31/17)=21
2x=21-3(31/17)
2x=264/17
x=132/17
* a/b=a為分子,b為分母
2. b+a=38 ...(1)
a-2b=26 ...(2)
From (1),
b+a=38
a=38-b ...(3)
Sub (3) into (2), [can't sub into(1) because (3) is a solution of (1)]
(38-b)-2b=26
-3b=-12
b=4
Sub b=4 into (1),
4+a=38
a=34