5x + 3y + 1 = 0
3y = -5x - 1
y = -5/3x - 1/3.....slope here is -5/3. perpendicular lines will have negative reciprocal slopes. So the other line will have to have a slope of 3/5.
ax + 2y + 5 = 0
2y = -ax - 5
y = -1/2ax - 5/2
-1/2a = 3/5
-a = 3/5 * 2
-a = 6/5
a = -6/5
lets check this....it looks funny
-6/5x + 2y + 5 = 0
2y = 6/5x - 5
y = (6/5x * 1/2) - 5/2
y = 3/5x - 5/2.....and the slope is 3/5
ok, then ur answer is : a = -6/5 <==