Question

Can you please help me with this and thank you

Answer 1

The length is 14 how I know it.

you don't know the value of the length so you will represent length by "X", you know you get the volume multiplying height by width by length so you multiple all the values.

6×10(X)=840

10(X)=140

X=14

if you do not know this process in detail leave it in the comments and I'll explain it, for now the value of length will be 14