book Maths Puzzle Numerical Ability

Solve for x,y in the following system of three
x + 10y + z = 52
5x + y + 4z =15
x + 2y - 3z = 12

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book Other Question

eight to the second power minus three to the second power times six divided by eighteen plus nine The teacher is asking me to simplify this complex fraction.
1
----- +1
a
--------
1
---- -1
a

I am struggling with this problem so any help is appreciated.