Question 315239
Your equation is incorrect.
Think about it this way.
The T-shirt company charges the $50 whether you sell zero t-shirts or 1,000,000 t-shirts. 
So that cost is a constant, that is, it's NOT a function of t, the number of shirts. 
The per-shirt cost is $6.50/shirt. The units should be the give-away. It has units of $/shirt and it needs a # of shirts multiplier to give you units of cost($).
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$=($/shirt)*shirt
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OK, so now the function looks like,
{{{F(t)=50+6.5*t}}}
Now work through the problems and see how it goes. 
Re-post if you get stuck.