3x-6+10x+15=35
10x+3x=35-15+6
13x=26
x=2
Answer: The standard error of the mean = 0.5
Step-by-step explanation:
Given : Sample size : n = 144
Standard deviation : 
The formula to find the standard error of the mean is given by :-

i.e. 
Hence, the standard error of the mean =0.5
Can you provide more information? Where would the fire be? What type of fire?(wildfire, controlled fire) What is the purpose of the fire?
Are you trying to find n?