Question

... the product of 86 and the depth of the river ...  into variable expression

Answer 1

product means multiplication either
86n
or

86*n which are the same thing

Answer 2

A variable expression is an expression that contains variables or letters in their simplest form. If we assign real values to the variables, then we can evaluate the variable expression for that real value.

Since we do not know the depth of the river, we assign a variable to it.

We let the variable
`d`, represent the depth of the river.

Then, the product of
`86` and the depth of the river
`=86* d=86d`