Paul Samuelson's textbooks define economics as the study of how societies use scarce resources to produce valuable commodities and distribute them among different people — scarcity (Robbins) + welfare and distribution (Marshall) + growth (Smith), in one sentence. That is the definition to write.
Law student's note: the definition wars matter to lawyers because each implies a legal world: wealth-definition economics asks law to protect property and contract; welfare economics asks law to correct markets (tort, regulation); scarcity economics asks law to allocate rights where markets cannot (emissions, spectrum, water) — the entire field of law-and-economics grows from these roots.