The core of this formula is INDEX, which is simply retrieving a value from C6:G10 (the "data") based on a row number and a column number.
=INDEX(C6:G10, row, column)
To get the row and column numbers, we use MATCH, configured for approximate match, by setting the 3rd argument to 1 (TRUE):
MATCH(J6,B6:B10,1) // get row number MATCH(J7,C5:G5,1) // get column number
In the example, MATCH will return 2 when width is 290, and 3 when height is 300.
In the end, the formula reduces to:
=INDEX(C6:G10, 2, 3) = 1800