lets x is price of noble fir trees and y - is price for douglas fir trees
students got 5x+3y=420$ on friday
and 12x+9y=1080
now we need just solve the system of equations
5x+3y=420 - multiply by 3
12x+9y=1080
15x+9y=1260
12x+9y=1080
substract second equation from first
15x-12x=1260-1080
3x=180
x=60$ - the price of noble fir trees
put value x=60 in first equation 5x+3y=420
5*60+3y=420
300+3y=420
3y=120
y=40$ - price of Douglas fir trees
Answer: Noble fir trees cost $60 and Douglas fir trees cost $40