If you have a sketch, that should be simpler.

The properties of a parallelogram is such that the sides AB and CD are parallel, meaning that AB and CD have the same gradient.

Same for BC and AD; they have the same gradient.

So:

1. Find the gradient of AB. (this is also gradient of CD)

2. Find the gradient of AD. (this is also gradient of BC)

3. Find equation of line through D having gradient AB.

4. Find equation of line through B having gradient AD.

5. Solve simultaneously for those two lines. The intersection is where point C is.

There is a shorter, visual way using vectors, but this makes a diagram a must, if you cannot picture the points in your head.

Vector AB = (10, 1)

So, C + (10, 1) = (-3, -4)

So, C = (-3 -10, -4 -1) = (-13, -5)