The key here is that if two lines, l1 and l2, with slopes, m1 and m2, are perpendicular then
m2 = -1/m1
they are perpendicular and what is m+b
Rom has given you a hint, he has not answered your question as you should be able to take his hint and answer the question yourself.
I'll give you some more hints.
What is the gradient of 5x+8y=-9 ?
You may need to rearrange it and put it in the form y=mx+b where m is the gradient and b is the y intercept.
There are other ways to do it but this is the one most taught.
Then follow Roms hint. i.e The gradient of the other line is the negative recoprocal of the one you have.
If you cannot at least find the gradient of the line then you are not ready to answer this question.
And we are here to help you learn not to supply you with a cheat answer.