Determinants

Let us consider a system of two linear equations for two unknowns, say ⋆ a x + b y = u , c x + d y = υ . https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315136547/a57ae1f0-6f59-46bd-bbdd-35a521b486a1/content/eq712.tif"/>