解:(1)$[2\frac{1}{5}] = 2,$$[-3.6] = -4,$$[-7] = -7,$原式$= 2 + (-4) - (-7) = 5;$
(2)$\{2\frac{3}{4}\} = 2\frac{3}{4} - [2\frac{3}{4}] = \frac{3}{4},$$[-2.4] = -3,$$\{-6\frac{1}{4}\} = -6\frac{1}{4} - [-6\frac{1}{4}] = \frac{3}{4},$原式$= \frac{3}{4} - (-3) + \frac{3}{4} = \frac{9}{2}$