book Maths Puzzle Numerical Ability

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!

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book Other Question

(12X+4)6Y+(3X+4X)=0 The half-life of Bismuth is 5 days. Then 75% of an original amount will cease to be radioactive in 10 days. I believe that this one is true. Am I right?