exam Maths Puzzle Numerical Ability

This is a last Subsitution question, but this one says solve by either ;addition or subtraction.

The problem is:
12a-5b=18
15a-5b=30

Please and Thank You=)

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exam Other Question
I couldnt figure this one out (factoring)

y^4-14y^3+49y^2


THANKS Please, please help? I need to solve:


by addition: 5x - 3y = 13 4x - 3y = 11

by graphing:x - y = 5
x + y = -1