$(-1 \frac{3}{4}) + \frac{15}{3} - 4.9 = -2.65$

$-\frac{3}{7} - (-0.4) - 1 \frac{3}{4} = -\frac{249}{140} = -1 \frac{109}{140}$

$-4 - (-\frac{6}{5}) - 2.3 = -5.1$