Write a problem that has two parts. then solve it by finding the whole

Sally wants to buy some groceries. She wants to buy 2 kg of potatoes and 1 kg of onions. 1 kg of onions cost $5 but the paper that shows price of potatoes got lost. However, Sally remembers that few weeks ago she bought 3 kg of potatoes for $12. How much did Sally spent buying groceries?

Part 1:
First we need to find how much 1 kg of potatoes cost.
3*x = 12
x = 4
1 kg of potatoes cost $4

Part 2:
Now we need to find how much did she spent.
M = 4*2 + 1*5 = $13.