The answer is the 3 option
Use pemdas : so do parenthesis first 15-8, then multiply the answer by 6. OR u can use distributive property & multiply 6 by 15 & 8
Combine like terms-- you can combine the k's:

Now you have:

Simply divide both sides by 2.5 to find the value of k:
She practiced 630 hours in all because you multiply 18x35