Given that total number of pounds of grass seed needed = 9
Let number of pounds of Bermuda seeds used = x
Let number of pounds of Fescue seeds used = y
Then sum of both gives equation:
x+y=9
or y=9-x...(i)
Given that cost of 1 pound of Bermuda seed = $4.80
Then cost of x pound of Bermuda seed = $4.80x
Given that cost of 1 pound of Fescue seed = $3.50
Then cost of y pound of Fescue seed = $3.50y
total cost will be 4.80x+3.50y
Given that cost of 1 pound of mix seed = $4.02
Then cost of 9 pound of mix seed = $4.02*9
combining all three parts, we get equation:
4.80x+3.50y=4.02*9
4.80x+3.50y=36.18...(ii)
plug (i) into (ii)
4.80x+3.50y=36.18
4.80x+3.50(9-x)=36.18
4.80x+31.5-3.50x=36.18
4.80x-3.50x=36.18-31.5
1.3x=4.68
x=3.6
Now plug x=3.6 into (i)
y=9-x=9-3.6=5.4
Hence final answer is given by:
Let number of pounds of Bermuda seeds he should buy = 3.6 pounds
Let number of pounds of Fescue seeds he should buy = 5.4 pounds
. Solving for (5+X)(5-X)=7 by multiplying the factors and subtratcting 7 on both sides gives the equation 
, so 
