You can use an elimination or a substitution.

Elimination:

Code:

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x + y = 10
 x - y =  6
2x + 0 = 16

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or

Code:

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x +  y = 10
-x +  y = -6
 0 + 2y =  4

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Substitution:
x - y = 6
x = 6 - y

Plug in the first equation:
(6 - y) + y = 10

There are many ways to solve simultaneous equations. You just do the one you're more confident with.