It will be very useful if you draw your own picture for this. Start by drawing an arbitrary parallelogram. the letters ABCD must label the vertices in order either clockwise or counter clockwise. It does not matter which is which, but this essentially guarantees that A is opposite C and B is opposite D. Once you have this labeled you'll need segment DB in your picture so that you have angle DBC to label. Now label the angles you know. You know angle A=75 and C is opposite A. In parallelograms opposite angels are congruent so C=75 as well. Now we know two angles of that triangle because DBC is also in that triangle. We know triangles Sum to 180 so CDB=180-75-60=45

Angle ADB is alternate interior to DBC they are congruent, therefore ADB=60