Area:
The expression for area would be length x width, therefore the expression would be (3x + 1)(2x -7)
Perimeter:
There are many expressions for perimeter. Here are some of them:
Length + width + length + width
2length + 2width
2(length + width)
Let's use the first one, for example. The expression would be (3x + 1) + (2x - 7) + (3x + 1) + (2x -7).