calculate the concentration of alkaline phosphatase (in units/L) when 0.5 mL of the enzyme catalyses..?

Here's a calculation, step-by-step:

The definition of 1 unit of enzyme activity is 1 µmol substrate converted per min (this is a given, what you need to have memorized).

1) 0.5 ml of your alk phos converted 450 nmol in 10 min.  

2) 450 nmol / 10 min / 0.5 ml = 45 nmol / min / 0.5 ml = 90 nmol / min / ml.

3) Convert nmol to µmol by dividing by 1000 [1000 nmol in 1 µmol]: 0.090 µmol / min / ml.

4) They want it in terms of activity per liter, so multiply by 1000 again [1 L = 1000 ml], which gives you an activity of 90 µmol / min / L = 90 units/L.

NOTE: Steps 3 & 4 can be combined if you understand that to convert nmol/ml to µmol/L, the unit conversion is done by just dividing both numerator and denominator by 1000, i.e. 1 nmol/ml = 1 µmol/L--in this case, 90 nmol/ml = 90 µmol/L.