First, see that the right hand side of equation (2) is a polynomial and thus continuous. This implies that this can only change sign if this firstly goes by zero. Therefore, if the derivative will change signs it will do thus at v = 50 but no guarantees that it will and the only place that it may change sign is v = 50. This implies that for v > 50 the slope of the tangent lines to the velocity will have similar sign. Similarly, for v < 50 the slopes will also have similar sign.  The slopes in these ranges may have and/or probably will have various values, although we do know what their signs should be.

Let's start through looking at v < 50. We saw previous that if v = 30 the slope of the tangent line will be 3.92 or positive. Thus, for all values of v < 50 we will have positive slopes for the tangent lines. Also, by equation (2) we can notice that as v approaches 50, all the time staying less than 50, the slopes of the tangent lines will approach zero and thus flatten out. If we move v away from 50, staying less than 50, the slopes of the tangent lines will turn into steeper. If you want to get a concept of just how steep the tangent lines become you can all the time pick exact values of v and calculate values of the derivative. For illustration, we know as at v = 30 the derivative is 3.92 and thus arrows at this point must have a slope of around 4. By using this information we can here add in several arrows for the region below v = 50 as demonstrated in the graph below.

Here, let's look at v > 50. The first thing to do is to determine if the slopes are negative or positive. We will do this similar way that we did in the last bit, that is pick a value of v, plug it in (2) and notice if the derivative is negative or positive. See that you must NEVER suppose that the derivative will change signs where the derivative is zero. This is easy adequate to check so you must always do so.