<PRE><B>Question 87535</B>
1. {4x+y=13
   {2x-y=5 
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Solving with substitution:
Solve the 2nd equation as follows:
y = 2x-5
Substitute into the 1st equation to solve for x:
4x+(2x-5) = 13
6x=18
x=3
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Substitue into y=2x-5 to solve for y:
y=2(3)-5
y = 1
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Check the answers in 4x+y=13
4*3 + 1 = 13 
13 = 13
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2. {2x+4y=6
   {5x+10=15 
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Multiply 1st by 5 and 2nd by 2 to get:
10x+20y=30
10x+20y=30
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This shows that each of the equation is a multiple of the other.
The solution is all points on the line where
y = (-1/2)x+(3/2)
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3. {x+y-z=-1
   {-4x-y+2z=-7
   {2x-2y-5z=7

These are too messy to work online but using matrix methods
I can tell you the solution is:
x=3 ;y = -3 ; z = 1
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Cheers,
Stan H.</PRE>