Question 283890
First of all the brand names are only going to lead to confusion so let's change that.
let call brand 391 t for three
and brand 176 o for one 
what do we know?
brand o sells for $3.00 lb
and she has 2 lbs of brand o so o=2
brand t sells for 1.74 lb
but we don't know how much she has of brand t . We will call it t
We want to sell a mix (o+t) for 2.58.


So how do we set up an equation?
the individual nuts have to be equal to the mix
What do we want to know?How many lbs of t are we mixing with o
to get o+t
so we are dealing with selling price.
the o selling price is 
2*o where o =300 
I am going to convert everything to cents so we don't deal with decimals.

2*300+t*174=(2+t)*258
solve for t