(a) To figure out what the deer population is on the graph (I am assuming that the vertical axis, also known as the y-axis, has "Deer Population" and the horizontal axis, also known as the x-axis has "Years") you can find where 2003 is on the x-axis and then draw a line straight up on the graph to find where the line is at in 2003. Then you draw a horizontal straight line to the y-axis from that point to find the number of deer for 2003.
(b) A function that would model the deer population is y=mx+b. Y is the population of deer, or in your case "n(t)". m is the value of the slope of the graph, which can be found by taking two points on the graph and using the equation (y2-y1)/(x2-x1). x is "t" in your function as it would represent what year. Which leaves b. b is the intercept value, which means where x (t) equals zero- you would use the y-value, or number of deer at that year as your number for b.