GMAT Math

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

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

y = -48

By mistake u have flipped signs when adding

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

2y = 48
y=24

24+3z=51
3z=27
z=9

Regards,
Cramya

Thank you!

1. y/3 + z = 17

First to get rid of the denominator, multiply by 3. That will also mean multiplying all the values by 3.

3(y/3) + 3z = 51
so
3y + 3z = 51
y - 3z = -3

This is already setup for us as simultaneous equation so
2y = 48
y = 24

24 - 3z = -3
-24 -24
-3z = -27
z = 9

24 - 27 = -3