Assuming those 'x's mean multiply, the missing term is 5
So we go from this
(2x4) x ?? = 2x(4x5)
to this
(2x4) x 5 = 2x(4x5)
where the question marks have been replaced with 5
The rule to use is the associative property of multiplication. It's the idea where we can group three values in two ways to multiply in any order. Either we do 2 times 4 first, or we do 4 times 5 first. It leads to the same answer. This idea is similar to the commutative rule but it's not exactly the same.