How do set up a substitution equation like 3x-y=5, x-4y=-24

Well to set up a substitution equation, you need to make sure you have something you can sub in that's identical in both equations.

we have

3x-y=5

x-4y=-24

what's in common? x and y.

take one of the equations and solve for x or y like so (i'm doing y)

3x-5=y

then substitute the whole 3x-5 equation for y in the second equation. it is quite similar to how you would normally substitute a number like if y=5, then you substitute 5 for y in the second equation, but instead the "5" is an equation "3x-5"

x-4y=-24

x-4 (3x-5) = - 24

Now that you got it set up, you just need to solve for x and once you got x value, plug the x value in the original equations to find y.