Perpendicular lines

Two lines ax + by = 5 and 4x - 5y = 3 are peroendicular. What is the relationship between a and b?

Lines y = mx + b is perpendicular to y = m'x + b' if mm' = -1 (to find a line perpendicular to m, flip the sign and flip the number to get m' = -1/m)

First get the equations in standard form so that you can find y. Line 1 has an m value of -a/b. Line 2 has an m value of 4/5, so if they are perpendicular, (-a/b)(4/5) = -1, or a/b = 5/4