What are some advantages to writing a direct variation equation rather than solving a problem using a proportion

When i add money am i supposed to make it like this 7+12 or 7.00+12.00 because i got 19 dollars for my answer

Acabinmaker buys 3.5 liters of oat varnish the varnish cost $4.95 per liter what is the total cost of 3.5 liters

Multiply and simplify. (x - 4) (x^2 – 5x – 6) i will mark brainliest if your answer is right. show how you got the answer. if your answer is correct but does not show how you got the answer you will not be marked brainliest.