What is the molecular weight(MW) of the protein? Depending on the MW, convert the mg/mL unit into mole specific unit (umole/nmole etc), then apply specific activity equation.
Yes, surely the question is vague. As Liger has referred, one unit of an enzyme is the amount of enzyme that converts one micromole of the substrate into it's product under specific condition of reaction i.e. Temp, pH etc. This if 25 micromole of substrate is converted by 0.9 mg/ml of enzyme, it's about 0.036 mg of enzyme that shall convert 1 micromole of substrate.