I'm stuck on this problem:

y/3 + z = 17 and y - 3z = -3

Here is how approached it:

3(y/3 + z) = 17 * 3

y + 3z = 51
y - 3z = -3
y = -48

Now I plug in -48 into the second equation.

-48 - 3z = -3
+48

-3z = -3 + 48
-3z = 45

-3z = 45
 -3      -3

z = -15

Where am I going wrong?
The answer is stated as:

z = 9

y = 3(8) = 24