Let x be the number of kg of peanuts and y be the number of kg of almonds. So 3x is the amount of money we're spending on peanuts and 9y is the amount of money we (or somebody) is spending on almonds. So the total spent is 3x + 9y and the amount of mixed nuts we're getting (or selling?) is x + y. So ...

3x + 9y = (7.5)*(x+y)

You could graph this as a line. Any number of possible combinations. But we're told to buy (or sell?) four kg of peanuts, so

3*4 + 9y = (7.5)*(x+y)

12 + 9y = (7.5)* (4 + y)

Can you solve that for y?