book Maths Puzzle Numerical Ability

I can't seem to make out how to do this problem. it is so confussing. the problem is:
-7.5xs-0.9=-5.7x+4.5

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book Other Question

Given the values of the 4 sides of a trapezium, find the length of the diagonals.
The length of the parallel sides are 10cms and 16cms. The other 2 non-parallel sides measure 12cm Sally has $32,000, she invested part at 9% return and the remainder at 11% return. Her total return was $3200, what is the investment breakdown? How do you set this equation?