= t^3 - 6t^2 - 15t - 5.
Xo=0^3 - 6*0 - 15*0 - 5 = -5 cm @ t = 0.
X=7^3 - 6*7^2 - 15*7 - 5 = -61 cm @ 7s.
V = D/t = (X-Xo)/t.
V = D/t = (-61-(-5)) / 7 = 56 / 7=-8cm/s
A particle moves in a straight line so that its position (x cm) from a fixed point O at time (t seconds) is given by x= t^3-6t^2-15t-5
Find the average speed of the particle in the first 7 seconds
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