exam Maths Puzzle Numerical Ability

i do not understand this show me please?
(2r-3s)(4r^2+6rs+9s^2)

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solve 4A/3=1/2b^2c^2d for c

thank you PQR is an isoceles triangle such that QP = QR = 13cm and the altitude PS = 12cm. Find the length of PR.