There is a couple of things I notice here. Do you see how the ( is italicized before the [ ? That is letting you know it is treating the ( as part of the units. The easiest solution would be to add a * 1 before each of the units. This will multiply you scalar by a unit value of each. The alternative would be to put the units inside the (). I would also double check your order of operations. Do you want (15^3)/2 or do you want 15^1.5? I would assume 15 [m/s] ^ 1.5 to obtain units of pressure.