book Maths Puzzle Numerical Ability

i need help on this problem:
2.4 + 3[6.8 -(-1.9) + 0.48]
i tried it but i got it wrong , i got : 4.887

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book Other Question

How does the problem (3x to the fourth power y to the third power)(4x to the fourth power y) work. Square root of 18 + Square root of 12.
I dont have a square root button on my computer..sadly, so hopefully you see what im asking.