A graphing calculator shows the rocks are at the same height 1.5 seconds after they are released.
That height is 3.975 meters.
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f(x) = g(x)
-4.9x^2 +15 = -4.9x^2 +10x
15 = 10x . . . . . . . . . . . . . . . . . . add 4.9x^2
1.5 = x . . . . . . . . . . . . . . . . . . . divide by 10
f(1.5) = -4.9*2.25 +15 = 3.975
That height is 3.975 meters.
_____
f(x) = g(x)
-4.9x^2 +15 = -4.9x^2 +10x
15 = 10x . . . . . . . . . . . . . . . . . . add 4.9x^2
1.5 = x . . . . . . . . . . . . . . . . . . . divide by 10
f(1.5) = -4.9*2.25 +15 = 3.975