This isn't an answer, rather a suggestion. I would use the website desmos to help me. But for future reference I can help you.
f(x) = - x+6 is similar to y=-x+6.
Simply plug in random numbers into the variable x (preferably ones that could fit on your paper like 0,1,2,3...)
Answer:
JUST USE THE CALCULATOR DUHHH!!!
ANSWER- 23 , 76
Explanation- just did it :)
First, multiply the 7 on both sides.


Finally divide by -1 on both sides to get t = -7/15 as your answer.