Net Exports
Assume that net exports are independent of the level of GDP. In this case, equilibrium GDP can be written as Ye = C + Ig + Xn = a + bYe + Ig + Xn. Solving for Ye we obtain: Ye = 
x (a + Ig + Xn). We note that investment and net exports enter the solution in a symmetric fashion. Like investment, positive net exports increase equilibrium GDP while negative net exports decrease it. Likewise, we see from inspection that 
= 
, which was previously identified as the "multiplier."
More generally, Xn is a function of prosperity abroad, or foreign incomes (Yf), tariffs (t), the international price or exchange value of the dollar (P$), and the domestic price level relative to foreign prices (P/Pf): Xn = Xn(Yf, t, P$, P/Pf). Substituting this into our relationship for Ye we obtain a more complete picture of equilibrium GDP. Ye = x (a + Ig + Xn(Yf, t, P$, P/Pf)). Using the function of a function rule, we can differentiate Ye with respect to foreign incomes, tariffs, the exchange rate and the domestic/foreign price ratio to obtain the following results:
= x 
= x 
= x 
= x 
That is, first determine the impact on net exports of a change in either foreign income, tariffs, the exchange rate, or the domestic/foreign price ratio. Then multiply that change in net exports by the multiplier to calculate the effect on equilibrium GDP.