6.4/3.5, OR about 1.83
First job is to isolate the y.
3.5y=6.4x+7
Now divide both sides by 3.5 to get the equation into y=mx+b form.
That leaves a rounded equation of y=1.83x+2
In y=mx+b, the m is the slope. This means the slope of this line is 1.83, or 6.4/3.5