Ido not understand how to do this 251/4÷-3/8

When dividing fractions, you switch the denominator and numerator on the second fraction, and change the divided sign to a multiplication sign.

So,

251 -3
/
4 8

becomes

251 8
x
4 -3

251 x 8= 2008
4 x -3= -12

Now, divide 2008 by -12.
2008 does not go into -12 evenly (the answer would be -167.333333333333. The 3s would go on forever)

2008/12 is equivalent to 502/3

So, 251/4 divided by -3/8 is around 167 1/3, or 502/3