Solving for unknown variable

Actually the unit of Q does not matter when the question is just about solving the equation. The unit of the solution Va is unit_of_Q/m^2.

Its quite likely that Q is volumetric flow rate with unit m^3/s and so Va ends up as velocity (e.g. unit m/s)

The equation in question is quadratic, so we have to expect two solutions.

A plot can help to determine suitable guesses or ranges to be used in the "root" function.

This solution works OK in Prime Express

As can be seen neither f(V1) nor f(V2) are exactly zero but only close. This is due to numerical inaccuracy and round off errors we get when we use numeric methods working on variables stored in IEEE format.

As already written, from the picture sent by the OP he/she is using Prime Express and so can't use a solve block.

Furthermore its questionable where you get your guess values from - its guess and check as well and in that case its always a good idea to plot and look where the solutions are to be expected..

Only a symbolic solve would give you both solutions without having to know in which range to look for them.

But your approach getting both values in one go is possible to do in Express as well. The "root" command can be used with a vector as a guess as in your approach with the solve block and the "root" function can be vectorized.