1. Units

100.0 g x 1.00 cal/g x Celcius x 28.6 degrees celcuis

I need to multiply all of these and use significant numbers.
I think the number would be 3000, but how do I label it?

2. First, we should have it in a way that is visually easier to understand:

$(100.0g)\left(1.00\frac{cal}{g*Celsius}\right)(28. 6 \ Celsius)$

We first multiply the first two terms in the analysis:

$\left(100.\frac{cal}{Celsius}\right)(28.6 \ Celsius)$

Notice that the 100.0 lost it's 0 because 1.00 has 3 significant digits and therefore 100 also has to have three sig digs, since 100.0 has 4 significant digits, then we keep the decimal and lose the zero to get 3 significant digits. Now we multiply the other two numbers:

$\left(100.\frac{cal}{Celsius}\right)(28.6 \ Celsius) = 2860 \ cal$

You can use 2860 because both 100. and 28.6 have 3 significant digits, and 2860 has 3 as well, therefore the answer works.