equal: Syntax Error (Improperly Terminated String ')')19 20

Hello, 

I am trying to implement a formula where I will get a value from subtracting two fields ($prediscount_total_amout_c,$existing_arr_c) based on the conditions that another field on the layout $arr_yes_no_c,"yes".

I've added this formula:

 (equal($arr_yes_no_c,"yes"),subtract($prediscount_total_amout_c,$existing_arr_c)

however I receive this error: 

equal: Syntax Error (Improperly Terminated String ')')19 20
Could anyone explain what does the error mean and how can I fix it?
Thanks!
Ada