Complex sentence: Mainstream economic reasoning begins with the postulates of scarcity and rational agents (that is, agents modeled as maximizing their individual welfare), with the "rational choice" for any agent being a straightforward exercise in mathematical optimization.
Split into simpler sentences: Mainstream economic reasoning begins with the postulates of scarcity and rational agents (that is, agents modeled as maximizing their individually-given welfares). With the foregoing stipulations, the determination of the "rational choice" for any agent becomes a straightforward exercise in mathematical optimization.