Rhombus ABCD has perimeter 148, and one of its diagonals has length 24. How long is the other diagonal?

If you know that the area of the rhombus is 840 (see another of your questions), then you can find the other diagonal by using the formula:  Area  =  (diagonal 1) x (diagonal 2) / 2

--->   840  =  (24)(x) / 2

--->  1680  =  24x

--->  x  =  70