How can you tell if shapes are similar or not?

Tomas is making trail mix using granola and walnuts. he can spend a total of $12 on the ingredients. he buys 3 pounds of granola that costs $2.00 per pound. the walnuts cost $6 per pound. he uses the equation to represent the total cost, where x represents the number of pounds of granola and y represents the number of pounds of walnuts. he solves the equation for y, the number of pounds of walnuts he can buy. answer- b tomas added 6 to both sides of the equation instead of subtracting 6. just took the test

How many credit hours will a student have to take for the two tuition costs to be equal? round the answer to the nearest tenth of an hour.

How do you use the elimination method for this question? explain, because i really want to understand!