If x-y =8 find the value of y when x=3

y = -5

Substitute in 3 for x.
x - y = 8                            Substitution
3 - y = 8                            Subtract 3 from both sides
-y = 5                                Divide by -1 to both sides
y = -5                                Answer!

You can also check your work.
x - y = 8       
3 - (-5) = 8                  
3 + 5 = 8                        
8 = 8