I am not sure what substitutions are, but to solve, you must isolate y. Like so:
-5x+y=-3
Subtract -5x and two negatives make a positive so u add 5x so that y will be Alone, you must add 5x to the -3 as well because in an equation, whatever happens to one side, u must do to the other, so your equation so far should look like this:
Y=5x-3
That's your answer for the first one, here's the second
3x-8y=24
Subtract 3x
-8y=-3x+24
Divide by -8 to isolate y
Y=3/8x+24
Hope I helped
