SOLUTION: how do i solve the system using the substitution method for these two equations? {{{ -x-4y= -3 }}} {{{ 2x+y=15 }}}

Question 503170: how do i solve the system using the substitution method for these two equations?

Answer by Maths68(1474) (Show Source): You can put this solution on YOUR website!
-x-4y= -3 ...........(1)
2x+y=15 .............(2)
Take (1)
-x-4y= -3
-x=4y-3
Multiply by (-1) both sides of above equation
x=-4y+3
substitute the value of x in (2)
2(-4y+3)+y=15
-8y+6+y=15
-7y=15-6
-7y=9
Divide by -7 both sides of the above equation
-7y/-7=9/-7
y=9/-7
Put the value of y in (1)
-x-4(9/-7)= -3
Multiply by -7 both sides of the equation
7x-36=21
7x=21+36
7x=57
x=57/7
Solution set (57/7, -9/7)
Check
Put the values of x and y in either of two equations (1) or (2)
-x-4y= -3
-57/7-4(-9/7)=-3
-57/7-4(-9/7)=-3
Multiply by 7 both sides
-57-(4)(-9)=-3 * 7
-57+36=-21
-21=-21
2x+y=15
2(57/7)+(-9/7)=15
Multiply by 7 both sides
2(57)-9=15*7
114-9=105
105=105
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