How many properties of parallelograms are there?

The answer is six.

Step-by-step explanation:  There are six important properties of a parallelogram. Please see the attached figure. The properties are as follows :

(i) The opposite sides of a parallelogram are parallel and congruent, i.e., PQ║RS, QR║PS, PQ ≅ RS and QR ≅ PS.

(ii) The opposite angles of a parallelogram are congruent, i.e., ∠P = ∠R and ∠Q = ∠S.

(iii) The diagonals of a parallelogram bisect each other, i.e., OP = OR and OQ = OS.

(iv) Consecutive angles of a parallelogram are supplementary. For example, ∠P + ∠Q = 180°.

(v) If one angles of a parallelogram is 90°, then all the angles are 90°.

(vi) Each diagonal of a parallelogram separates it into two congruent triangles. For example, ΔPQR ≅ ΔSQR.

c.

step-by-step explanation:

she should of added 9^2 and 12^2. that is because it is a^2+b^2=c^2. c^2 is the hypotenuse and should always be bigger than the other two numbers. which is why she should of added 9 squared and 12 squared together instead of 15 squared.