book Maths Puzzle Numerical Ability

how would I solve this problem to simplify for like terms?

3(4x+6)+7x

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book Other Question

Solve the system of equations for x and y by the addition method

2x-4y+3z=17
x+2y-z=0
4x-4-z=6

Eliminate 1 variable at a time I tried this exponent problem: (-9c2)3 with c=1/3. I got -1. Is this correct?

Also I did (ac)2 with a=2 and c=1/3 and got 4/9. Am I doing it right? Thank you. I am trying really hard to get the hang of exponents. R