Reaction has a theoretical yield of 56.9 g. when the reaction is carried out, 36.6 g of the product is obtained.

I'm assuming you need to know the percentage yield of the reaction

To calculate the percentage yield = (actual yield x 100%) / predicted yield

actual yield is 56,9 g

predicted yield is 36,6g (is the amount that's expected if nothing had got lost)

(56,9 x100) / 36,6=

= 155%