We have a=2; b=-1; c=-15;

Which means ac=-30;

Find factors of -30 that add up to -1;

In this case, 5 and -6;

Then, draw your box:

| 2y^2 ....| ******|

| ********| -15 **|

Add the numbers you found as y multipliers;

| 2y^2 | 5y |

| -6y...| -15 |

Then, find the common factors of each row and column to get;

............2y........+5........

y....... | 2y2^2 | 5y |

-3.......| -6y.....| -15|

You now have the following (y-3)(2y+5);

Try it yourself to see if it works.