To make your formula neater:

Acceleration is equal to
,
so when you multiply it by time, you will get a velocity. In fact, you would get the change in velocity, as a*t signifies the acceleration of the object over the time, aka the change in velocity.
The initial velocity plus the change in velocity will equal the final velocity, irregardless if the change in velocity is positive or negative.
Therefore,

Just like the water cycle, rocks undergo changes of form in a rock cycle. A metamorphic rock can become an igneous rock, or a sedimentary rock can become a metamorphic one. Unlike the water cycle, you can’t see the process happening on a day-to-day basis. Rocks change very slowly under normal conditions, but sometimes catastrophic events like a volcanic eruption or a flood can speed up the process. So what are the three types of rocks, and how do they change into each other? Keep reading to find out!
Answer:
400 N
Explanation:
Change of Kinetic Energy to Friction Wok
∆KE = W
½ x m x (v(5)² - v(3)²) = f x d
½ x 500 x (5² - 3²) = f x 10
250 x (25 - 9) = f x 10
25 x 16 = f
f = 400 N