The slope of equation is the coefficient "a"
Let 3x +4y = 5
We have isolate y
3x + 4y = 5
4y = -3x + 5
Diding both the sides by 4
y = -3x/4 + 5/4
Look,
a = - 3/4
Because, the equation od line is:
y = ax + b
Then, looking to us equation
a = - 3 / 4
Hope this helps