book Maths Puzzle Numerical Ability

(12X+4)6Y+(3X+4X)=0

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I am having trouble figuring out a problem I have for math homework:
8x(t)4y-4x(t)3-12x(t)2y

t= to the power

It reads: 8 x to the fourth y minus 4 x to the 3rd -12 x to the
1) 3x-2y=4
2x-5y=21

2) 4x+5y=-5
3x+y=21

3) 3x+y=7
3x+y=10

I cannot find the solutions to these systems. PLEASE HELP! I have to have them due soon!