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I've been set a physics assignment where I need to make a 2D projectile physics engine. Here's one of the tasks:

A trajectory in $2D$ for a projectile that has a launch velocity of 20 ms-1 and a launch
angle of $45^o$ . Time step = 0.01 seconds. The gravity vector is $(0.0, -9.81)$. Show the position vectors and velocity vectors on a console for demonstration purposes

I know where all the values are coming from, and I know their values, but I don't understand where I get the value of $a$ in the $v=u+at$ (this is the formula I must use)

My question is, how do I work out the value of $a$, so that I can perform the equation?

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    $a$ is given as the *gravity vector*. – 2017-01-06
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    Well, $a$ seems to be acceleration and if the projectile is lunched at constant speed, the only acceleration comes from gravity, i.e. $a = (0,-9.81)$. But, I might be wrong, you should clarify what variables are. – 2017-01-06

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$\vec{a}=(0,-9.81)$. The horizontal acceleration is zero which is given and the only acceleration on the projectile is the acceleration due to gravity which is also given. You will have a horizontal and vertical (sometimes $x,y$ or sometimes $i,j$) velocity and acceleration.