Substitution: y-2x=-6 and 5x-y=9

y-2x=-6 and 5x-y=9   

step 1: isolate y in the first equation

y = 2x - 6

step 2:  substitute the y value on the 2th equation:

5x-(2x-6)=9

step 3:  solve the present equation:

5x - 2x + 6 = 9
3x = 9-6
3x = 3
x = 1

step 4: get the y value by replacing the value of x in y = 2x - 6:

y = 2*1 - 6
y = 2 - 6
y = -4

S={1, -4}

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