I'm probably making a silly mistake or Wolfram Alpha is lying to me.

Question: Find the value of c and d.

Rearranged, simplified and multiply each equation by 2:

Now find the common multiple which in my case I will use 12:

Then add them and find what d is worth:

Now when I plug this back into the equation, I will use the first one:

What am I doing wrong? Sorry if this is the long winded way to do it.