Answer:
s = 648
Explanation:
given s varies directly as t² then the equation relating them is
s = kt² ← k is the constant of variation
to find k , substitute s = 128, t = 4 into the equation
128 = k × 4² = 16k ( divide both sides by 16 )
8 = k
s = 8t² ← equation of variation
substitute t = 9 into the equation for s
s = 8 × 9² = 8 × 81 = 648