That's a tricky one.  The correct answer is 399.5705.

The trick to solving a problem such as 23.5 x 17.003 is to multiply 235 x 17003.  You'll get 3995705.  Now, count up how many numbers there are after the decimal points in the problem.  In this case, there are four numbers after the decimal points (23.5 and 17.003).  Make your answer also has four numbers after the decimal point.

I recommend reading Volume 1 of the Learn Math Fast System.  That book will teach you how to multiply and divide any numbers.  Use the coupon code PT2026 to get 10% off your order.