If you just run the line

```
Solve[{x + y == 27, x y == 180}, {x, y}]
```

then everything runs fine and you find `{{x -> 12, y -> 15}, {x -> 15, y -> 12}}`

.

The error you see is because somewhere you've got a statement like

```
In[1]:= x + y = 27
During evaluation of In[1]:= Set::write: Tag Plus in x+y is Protected. >>
Out[1]= 27
```

Where the head of `x + y`

is `Plus`

which is a protected symbol and can't be naively given new definitions. It might be that you have some weird definition hiding somewhere, in which case, it's simplest to restart the kernel and try again.

The moral of the story is that `=`

(Set) is not the same as `==`

(Equal).