Laravel Two request value get decrement and insert in column

I'm New in Laravel I controller trying to get decrement (subtract) two input request value and pass result in ( balance_amount ) column.

example

input price request       100
input paid_amount request  50
result balance_amount      50

my Controller

 public function pay(Request $request)
    {
$request['price'] = $request->price;  
$request['paid_amount'] = $request->paid_amount;
$request['balance_amount'] = $request->balance_amount; 
 
    Installment::create($request);   
           }              

  return redirect( )->with('success',' PAID SUCCESSFULLY');

    }  

via Chebli Mohamed