Yes, you can do it using substitution. However, elimination would be better because you could just add the two equations and have y cancelled out immediately.
The answer would be f(x)=9 because when you substitue the 3 in for 'x' you then square the 3.
Hope this helps!
Let the two numbers be x and y
then x+y=9
x-y=3
2x=12
x=6
6+y=9
y=3
so the numbers are 9 and 3
1/12:1/8
1 in per 1/8min
12in per 1/8(12)
12in per 3/2min
12in per 1.5min
Answer 1.5 minutes.
P=$1600 + 320 (r) x 8 yr (t) (im probably wrong :P)