exam Maths Puzzle Numerical Ability

solve the equation by making an appropriate substitution:

(x)2/3 minus (x)1/3 minus 6 = 0

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exam Other Question

Mix a solution that is 70% acid with a solution that is 90% water to make 9L of a solution that is 30% acid. How much of each solution should you use? I have worked part of this problem, now I am stumped. How do I write sqrt of -108 as an imaginary number? The prime factors I came up with are 3*3*3*2*2.
Thank you so much