I'm supposed to use marked dots and set a formula for y.

I have the dots: (1;4) & (5;1)

we know that y=kx+m

So we get k by doing: y2-y1/x2-x1

k=1-4/5-1 = -3/4 = -0.75

So.. y = -0.75*x+m

Then you're supposed to put in one of the coordinates (either 1;4 or 5;1) to replace x and y so let's do that below, I'll use 1;4.

4 = -0,75*1+m

4 = -0,75 + m

m = 4 - (-0,75)

m = 4,75

So the answer is supposed to be: y=-0,75x+4,75 you'd think?

Wrong; the answers says it's y=-3x/4+19/4

Why do they put it like this? I see that 19/4 is 4,75 but what am I doing wrong in my calculations?
God I really suck at math & I'm pretty ashamed that I keep having to do these topics

fractions are exact answers. decimal places imply there is some error present. math courses really want you to use exact answers while science courses want you to use decimal places

You're not wrong. The answer you have and the answer the book has are the same, just in different forms.

The poster above is correct that in general leaving it in the fraction form is more precise...When you have a fraction that will continue on for infinite decimal places. In this case however, you're able to express the fraction as a decimal precisely, without having to drop out any numbers, so it is pretty silly that it is rejecting your answer..