So the put a lot of words to make this seem more complicated than it is. Your first equation involves the money. I’m going to use x to represent tacos and y to represent burritos.
3x+7.25y=595 would be your first equation because you know the price of each item already just not how many. The second would involve the twice as many burritos sold than tacos. So that would mean x+2 would equal y
Y=x+2
Hope this helps. If not I can explain it more in detail.
First step is to determine the valency of each of x and CaCO3 from the given compounds:
1- As for Li2CO3: we can deduce that the valency of lithium is one while that of CO3 is two
2- As for XCl3: we can deduce that the valency of chlorine is one while that of X is three
Second step is to write the required compound:
X : CO3 (elements involved)
3 : 2 (write the valency of each)
Then write the positive ion (X) followed by the valency of the negative ion (2) and then the negative ion (CO3) followed by the valency of the positive ion (3).
The final x carbonate is written as: X2(CO3)3
<u>Answer:</u>
<u>For A:</u> The
for the given reaction is 
<u>For B:</u> The
for the given reaction is 1642.
<u>Explanation:</u>
The given chemical reaction follows:

The expression of
for the above reaction follows:

We are given:

Putting values in above equation, we get:

Hence, the
for the given reaction is 
Relation of
with
is given by the formula:

where,
= equilibrium constant in terms of partial pressure = 
= equilibrium constant in terms of concentration = ?
R = Gas constant = 
T = temperature = 500 K
= change in number of moles of gas particles = 
Putting values in above equation, we get:

Hence, the
for the given reaction is 1642.
Molar mass Na = 23g/mol
46g = 456/2 = 2mol
1mol = 6.022*10^23 atoms
2mol = 2*6.022*10623
= 1.204*10^24 atoms