What is the syntax error in this short ruby block?

```
def do_this (a,b,c)
    puts 1.to_s, 2.to_s,3.to_s
    if a == 4
        do_this (1,2,3)
    end
end
do_this (4,5,6)

```

I get errors on the fourth and seventh lines, where "`do_this`" is called. 

The error is: `'unexpected ")", expecting "." or...' [...]`