4.5 mph (aka 4 and 1/2 mph)
If I were you, I would make the starting point (3,-6). From there, you will want to use the slope of -1/2 (go down 1 unit and to the right 2 units and draw a point)
When you divide two fractions, you do Keep, Change, Flip or KCF. So you keep the 3/5, you change the division sign to a multiplication sign, then flip the 3/15 to 15/3. So the new equation is 3/5x15/3. Then you simply the two sides so it becomes 1/1x3/1.....
Meaning the answer is 3.
A²+b²=c²
64+b²=196
b²=132
b=11.48912529
