二元一次方程點做？？

a= 18-3b ... (1)
then put (1) into 3a+5b=24
then 54-9b+5b=24
4b=30
b=15/2

then a= 18-3x(15/2)
a= -9/2

a+3b=18......(1)
3a+5b=24......(2)
由(1),a=18-3b......(3)
由(2),3(18-3b)+5b=24
54-9b+5b=24
54-4b=24
-4b=24-54
-4b=-30
b=-30/-4
b=7.5

將b=7.5代入(1),a+3(7.5)=18
a+22.5=18
a=18-22.5
a=-4.5
解是(-4.5,7.5)

a + 3b = 18
a = 18 - 3b

substitute a = 18 - 3b into the second equation.
3 (18 - 3b) + 5b = 24
54 - 9b + 5b = 24
-4b = 24 - 54
-4b = -30
b = 7.5

substitute b = 7.5 into a = 18 - 3b
a = 18 - 3 (7.5)
a = 18 - 22.5
a = -4.5

a + 3b = 18 -----(1)
3a + 5b = 24 -----(2)

從(1), a = 18 - 3b -----(3)

將(3)代入(2),
3(18 - 3b) + 5b = 24
54 - 9b + 5b = 24
-4b = -30
b = 15/2 (= 7.5)

代 b = 15/2,
a = -9/2 (= -4.5)