1. Platelets help the blood to clot. Clotting stops the blood from flowing out of the body when a vein or artery is broken. Platelets are also called thrombocytes. Red blood cells carry oxygen.
2. Use of stem cells in medicine
Cell therapies would use stem cells, or cells grown from stem cells, to replace or rejuvenate damaged tissue. Scientists also want to use stem cells to understand disease and find drugs that might treat it. Embryonic stem cells could be used to make more specialized tissues that have been lost to disease and injury.
To find the slope given two points, just solve the equation (y1-y2)/(x1-x2)=m. So (2-r)/(r-5)=0. To get the slope to be 0, you need only the numerator to be 0 (if the denominator is 0, the slope is undefined).