book Maths Puzzle Numerical Ability

Sorry if the subject is wrong!

My question is:

3x-y= -3/2
4x+y=5

My work:

4x+y=5
-4x -4x

From there I get really confused! Thanks for the help!

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book Other Question

How many ml of a 40% acid solution should be added to 100 ml of a 50% acid solution to create a 42% acid solution? The difference between 1/4 of a number and 1/6 of the same # is 18. Which equation shows the correct relationship?
1.1/4x-1/6x=18
2.1/4x=1/6x=18
3.1/4x+18=1/6x
4.1/6x-18+1/4x=18
5.1/6x-18=1/4x