A rectangle or a parallelogram is a polygon with only 2 congruent sides and 2 congruent angles.
Create a system of equations
x + y = 123
5x + y = 343
I use substitution
y = 123 - x
5x + 123 - x = 343
4x + 123 = 343
4x = 220
x = 55
Plug in
x + y = 123
55 + y = 123
y = 68
Check
5x + y = 343
5(55) + 68 = 343
275 + 68 = 343
343 = 343
So what you need to do is find the volume and divide it by 2.
