Will and Grant left school and walked eight tenths of a mile due south to Will’s house to pick up his prize pig Petunia. Will, Grant and Petunia then walked four
tenths of a mile directly east to Grant’s house where his pooch named Pinocchio
was waiting to join the party. The two boys, Petunia and Pinocchio then walked
DIRECTLY back to school. How much shorter was their walk back to school than the walk to Grant’s house?

So in this case, we have to replace the known value.
y=3
y=-2x+3

3=-2x+3

Then we leave our unknown value alone.
= x

In this case, our x value would be 0.

We check it...
3=-2(0)+3
3=0+3
3=3

So y=3 x=0

For the second one we have...
y=3x+2
y=-3x-4

For this we substitute the y in any of the equation...
3x+2=-3x-4

We move the unknown values to one side and the ones without unlown values to the other side...
3x+3x=-4-2

Then we solve
6x=-6

Then we leave the unknown value alone.
x=

Then solve for x.
x= -1

Then for our y value we return to one of the original equations and substitute the x value.
y=3x+2
y=3(-1)+2
y=-3+2
y=-1

y=-3x-4
y=-3(-1)-4
y=3-4
y=-1

So in this case we got that x= -1 and y= -1