How many solutions does each system have?

System I:
$\displaystyle{4}{x}+{3}{y}=-{1}\usepackage{cmbright}\Huge\$

System II:
$\displaystyle{5}{x}+{2}{y}=-{7}\usepackage{cmbright}\Huge\$
$\displaystyle{12}{x}+{9}{y}={21}\usepackage{cmbright}\Huge\$

$\displaystyle{15}{x}+{6}{y}={21}\usepackage{cmbright}\Huge\$