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A good way to approach this question is to first expand the brackets . This gives 12x² - 21x + 20x - 35 (remember to take the negative seven into account). We can then simplify this to give 12x...

5x2 – 2x – 3x2 + 6x – 21= 2x2 + 4x − 21

12² = 4² + x² [This is pythagoras' theorem (a² = b² + c²) ]
144 =16 + x ^2
128 = x²
11...

Answer: (x+7)/(2x)

d (3 + xy) = 4
d = 4/(3 + xy)