Let line $l_1$ be the graph of $5x + 8y = -9$. Line $l_2$ is perpendicular to line $l_1$ and passes through the point $(10,10)$. If line $l_

L1   =  5x + 8y   = - 9

In the form     Ax + By   = C......the slope of this line   =  -A/B

So....the slope of L1   =    -5 /8

So....the slope of perpendicular L2   =     8/5

So..... using the point (10,10)....the equation of L2  is

y  = (8/5) (x - 10) + 10    simplify

y = (8/5)x  - 16 + 10

y = (8/5)x  - 6

m = 8/5    and b  = -6

So....m + b  =   8/5  - 6  =   8/5  - 30/5     =     -22/5