Answer: -4/3
The slope of the given line is 3/4. It's simply the value in front of the x. To be more specific, the equation given to us is in the form y = mx+b where m = 3/4 is the slope.
Flip the fraction and then the sign
flip the fraction: 3/4 ----> 4/3
Then flip the sign: 4/3 ----> -4/3
In other words, we take the negative reciprocal
Notice how the original slope 3/4 and the perpendicular slope -4/3 multiply to -1