Question 193114
problem  1
1)  2x+3y=1
2)-3x+y=15,,,,,y=(15+3x)

subst  into  (1)
2x+3(15+3x)=1
2x+45+9x=1
11x=-44
x=-4

y=15+3x=15+3(-4)=15-12=3

check  by  subst  in  (1)  &(2)   ok

problem  2

1)  5r-s=5,,,,,s=-5+5r=5r-5
2)  -4r+5s=17
subst  into  (2)
-4r+5(5r-5)=17
-4r+25r-25=17
21r=42
r=2

S=5r-5=5(2)-5=5

check  in  (1) &(2)  ok

problem 3

1)x-3y=0,,,,x=3y
2)3x+y=7
subst  into  (2)
3(3y) +y=7
9y+y=7
10y=7
y=7/10

x=3y=3(7/10)=21/10

check  in  (1)&(2)  ok

problem 4
1) 1/2  (x) -2y=17
2) 2x+y=104,,,,,y=104-2x
subst  into  (1)

1/2(x) -2(104-2x)=17
1/2(x) -208+4x=17
4.5x=225
x=50

y=104-2x
y=104-2(50)=104-100=4

check  in (1) &(2)  ok