How to count distinct columns query? [closed]

I have to find the total number or records but count is based on distinct of multiple columns.

I have following lines in my query.

$query = $this->createQueryBuilder("j");
$query->select(
                "COUNT(DISTINCT 
                        'j.mark',
                        'j.model'
                    ) as total");

But this is giving me error:

[Syntax Error] line 0, col 76: Error: Expected Doctrine\ORM\Query\Lexer::T_CLOSE_PARENTHESIS, got ','

Can anybody please help me solve this issue?

Thank You.