Question 711371
You are stuck at multiplying the two binomials.


Arrange the terms on a grid  and fill in each row and column element.


-------------3t--------------------20
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.
.
.3t----------{{{9t^2}}}-----------------{{{60t}}}----
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.
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.20---------{{{60t}}}-------------------{{{400}}}---
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.



Next, just sum the resulting partial products.
{{{9t^2+60t+60t+400}}}
{{{9t^2+120t+400}}}


Next you deal with the still unfinished multiplication by  -7t.