How do I create an equation from this question?

I must assume that you are talking about a specific type of equation - your class might be focusing on one type of equation, so that would help for answering this. I will assume you mean a linear equation, and go from there. We are looking for an equation
that relates the height of the tree to the amount of fruit. So, lets let x=height, and y=lb of fruit. So, we have two points, right? (4,38) and (18,72). From these, you can find the slope of the line. (72-38)/(16-4) = 34/12 = 17/6 . I would keep this a fraction.
now you can use the point-slope form of a line to find the equation of the line you want. y-y1=m(x-x1) where x1, y1 are either one of the points you already have. y-38=(17/6)(x-4) gives you y-38=17x/6 - 34/3 so y=17x/6 +80/3 now put x=18 into that equation.
You will get 77.7, or you can round to 78 lb if you want a whole number.