IS curve:
\(Y = C(Y - T, i - \pi^e) + I(i - \pi^e, Y_{- 1}) + \bar G\)
where \(\pi^e\) is expected inflation and the endogenous variables are Y and i (why they are typically used as the axes). 
LM curve:
\(\frac{M}{P} = L(i, Y)\)
Putting these functions in matrices and utilizing Cramer's rule, we can find the impact of an increase in government expenditures on output (the government multiplier):
\(\frac{dy}{d \bar G} = (\frac{L_i}{(1 - c_y)L_i + (c_r + I_r)L_y}) > 0\)