book Maths Puzzle Numerical Ability

What is the answer to this question?
3x+2.4 = -5.1

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book Other Question

My homework problem is this:

X+3[2 5] [12 7]
[1 4] = [0 -8]

The numbers after the 3 are one matrix and the numbers after the equals sign is another matrix.
whatis 1/4 plus 2/5 plus 3/10?