At a candy store, erika bought 3 kilograms of cinnamon red hots and 1 kilogram of gummy bears for $21.00. meanwhile, irene bought 3 kilograms of cinnamon red hots and 3 kilograms of gummy bears for $39.00. what is the cost of one kilogram of each type of candy?

You have to make a system of equations.
lets make the cost of gummy bears per kg b and the cost of cinnamon red hots per kg c.

3c+1b=21
3c+3b=39 

-3c-b=-21 (multiply one of the equation by -1 so that we can cancel out c)
3c+3b=39

2b=18
b=9 dollars per kilogram

to find c you can plug the value of b back into one of the equation above and (3c+9=21 which lead to 3c =12 so c=4 dollars per kilogram.

I hope this helps. 

Hope this ; )