Verified answer

factor by grouping

(25t^3 + 25t^2) + (21t + 21)

25t²(t + 1) + 21(t + 1)

(25t² + 21)(t + 1) ← distributive property

takes practice, it do.

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25t^3 + 25t^2 + 21t + 21 = 0

t(25t^2 + 25t + 21 + 21) = 0

t = - 42

2. (25t^2 + 25t) = 0

5t(t + 5) = 0

5t = 0

t = 0

3. t + 5 = 0

t = - 5