exam Maths Puzzle Numerical Ability

Solve the system by substitution.
x + y = 12
y = 2x

This problem has me a bit confused and need some help. Thanks.

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exam Other Question

Multiply the fraction:

2x^2-x-3 3x^2-11x-20
-------- * -----------
3x^2+7x+4 4x^2-9

I am completely lost on this one....to many numbers or letters I am not sure. Thank you for the wonderful help :-) Add. 20 + (-15)