The supply-side effects of a tax cut arise because taxes act as (a stimulus/an incentive/a disincentive) to work, save, and provide entrepreneurial services.
The supply-side from from an increase in government expenditure arise because government expenditure increases the quantities of (public goods and services/transfers to the poor).
So, a tax cut increases people's (disincentive/incentive) to work, to save, and to provide entrepreneur services, all of which lead to an increase in aggregate supply and potential GDP.