This means that the problem is not solvable for the values you entered. A little background information on how the TI-83 solver works might help you to understand the problem.
Say, for instance, you enter 0=x^2 - 3 as an equation, and solve for x. Then your calculator calculates the values of x^2 - 3 for a large number of x'es. If it finds that this expression is positive for one value of x (say x1) and negative for another (say x2), then it knows that there must be a zero between x1 and x2. It continues to narrow this interval by calculating x^2 - 3 for even more values of x between x1 and x2, until it has an answer with the desired accuracy.
But what if you entered 0=x^2 + 3. This expression is always positive, so your TI-83 is unable to find a pair x1, x2 between which the sign of the expression changes. Thus it gives a "NO SIGN CH(A)NG(E)"-error, because the problem has no solutions (which you can easily see in this case).
In your problem, this may mean that you equation really has no solution. Another possibility is that you made an error when entering the formula. I know the TI-83 sometimes seems to have a strange calculation order when multiplying or dividing, so maybe it helps when you use some brackets around divisions to make clear to your calculator in which order the divisions must be performed.
PS: Now I also see why it can't find a solution. You equation is a square, which is never negative, so it's normal that your calculator cannot find a sign change.
To overcome this problem, note that (D * tan(a) -4.9 * D^2 / V^2 / cos(a) ) ^2 is zero when (D * tan(a) -4.9 * D^2 / V^2 / cos(a) ) [without the ^2] is, so try solving that one instead :)
I have got a TI-83 too.