1.\:\:substitution\:x+y=8,\:x-y=-4
2.\:\:substitution\:3x-4y=-30,\:3x+2y=6
3.\:\:substitution\:-4x+9y=9,\:x-3y=-6
4.\:\:substitution\:12x+13y=9,\:15x-23y=6
5.\:\:substitution\:-2a+3b=14,\:a-4b=3
6.\:\:substitution\:3x-7y=27,\:-5x+4y=-45
7.\:\:substitution\:3x-4y=100,\:\frac{x}{3}+\frac{4y}{3}=100
8.\:\:substitution\:y=3x-4,\:y=\frac{3}{2}x-2
9.\:\:substitution\:\frac{x}{2}-\frac{y}{2}=-1,\:x+4y=-12
10.\:\:substitution\:\frac{1}{2}x+\frac{1}{7}y=4,\:\frac{1}{6}x-\frac{2}{7}y=-1
