Think for a few seconds about the "best practice" process. You may either (1) substitute known values of variables then solve for the one unknown value, or you may (2) solve the original equation for the one unknown in terms of the other variables and then substitute the known values.
For example, π is defined as C/d, but we often start with C=πd or C=2πr rather than immediately substituting for C or D.
For this problem,
A = (1/2)bh
b = 2A/h
b = 2(450)/9
b = 100
Note: With this starting formula, I can easily say "twice 450 is 900, divided by 9 is 100." (converting to decimals is unnecessary, various algebra steps have already been accomplished). I can also perform a very quick check: a rectangle with h=9 and b=100 has area of 900; a triangle with those dimensions has half of the area.