There are 56 blades of grass in 1 in.^2. there 230 blades of grass 4 in^2 of the same lawn

To solve this we must know the linear model. first is solve for the slope of the linear model, y = mx + b
m = (y2 - y1) / (x2 - x1)
m = (230-56)/4-1) 
m = 58
next is to solve y intercept b, from y = mx + b, using y = 56 and x = 1 
56 = (58)1 +b  
b = -2 
so the equation is 
y = 58x - 2
calculate the number of blades of grass is 3 in². 
y = (58)(3) - 2 
= 172 blades of grass in 3  in².