Here is a problem from a worksheet I have..I'm having a little trouble with it. My teacher neglected to explain most of it:

Now, I know everything is done in m/s, so that's 100/9 m/s. I know what the problem is asking and everything..but I don't have any formulas that I can use (I don't think)...in this case, can acceleration be used the same as deceleration? Is it okay to just put it in an equation as a negative?