It is said that two opposite sides of the quadrilateral are parallel.

So, it can be Square, Rhombus, Parallelogram, or Rectangle.

It is not square and rhombus because two adjacent sides are unequal(given).

It is not rectangle, because if it is rectangle the diagonal length will be $$\sqrt{\ 3^2+4^2}=5^{ }$$

So, it is a parallelogram. 

We know that for a parallelogram $$d_1^2+d_2^2=2\left(a^2+b^2\right)$$.

$$d_1=1$$

a=3

b =4

$$1^2+d_2^2=2\left(3^2+4^2\right)$$.

$$1^2+d_2^2=2\left(3^2+4^2\right)$$.$$1+d_2^2=50\ \Rightarrow d_2^2=49\ \Rightarrow\ d_2=7$$.

The value of the expression $$(800 + 7n - n^2)$$ is $$(800 + 7*7- 7^2)$$=800+49-49=800