Factoring:: \(-16t^2+80t+21=t=-\frac{1}{4},\:t=\frac{21}{4}\)
We first factor \(-16t^2+80t+21\) into \(-\left(4t+1\right)\left(4t-21\right)\)
Next, \(4t+1=0 \longrightarrow t=-\frac{1}{4}.\)
And, \(4t-21=0 \longrightarrow t=\frac{21}{4}.\)
Since the value has to be maximum, the answer is \(\boxed{\frac{21}{4}}.\)
Oh, I think I'm wrong....
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