For the sake of showing work I will replace the empty space with an x like so...

To find out what x is you must cross multiply (aka butterfly)
***Image of this step is attached below
3*24 = 8*x
72 = 8x
Next divide 8 to both sides to finish isolating x. Since 8 is being multiplied by x, division (the opposite of multiplication) will cancel 8 out (in this case it will make 8 one) from the right side and bring it over to the left side.
72 ÷ 8 = 8x ÷ 8
9 = 1x
9 = x
To make these fractions equivalent the empty spot must be 9

Hope this helped!
~Just a girl in love with Shawn Mendes