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

I am comlpetley lost at how to begin to solve this problem
here is the question
4/3 = 8
-
b

the way it is written in the book is 4/3 divided by b equals 8
I could use some help on the following. The instructions say: Find the domain for the following function and write your answer using interval notation.

f(x)= inside swuare root is 2x+6

I would appreciate any help.
Thank You
Kay