What is the slope of a line
parallel to the line 4x – 3y = 16?

"Parallel" means the same, which means the slope you find. The formula for slopes is y=mx+b, so we need to put this equation in the right order for that. ('m' is the slope.)
 Add 4x to each side, and get -3y=16+4x. Rearrange it to get it in the right order (y=mx+b).
This is -3y=4x+16. So the number in front of 'x' is 'm', which is the slope. You never actually see 'm', it is represented by a number. So four is the slope.