Since you have y alone on one side of the equation......
Use substitution and plug in 1/2x-4 for y in the other equation.

Distribute

Combine like terms

Add 8 to both sides

Divide by 4
and 
Plug this value in for x

Simplify

Multiply everything by 8 and simplify

Divide.....

Your answers are:

Answer:
SE = 0.025
Step-by-step explanation:
We are given;
Sample mean; x¯ = 14.52
Sample standard deviation; s = 0.075
Sample size; n = 9
Now,formula for standard error of sample mean is;
SE = s/√n
SE = 0.075/√9
SE = 0.075/3
SE = 0.025
Mean is equal to sum/ number of values
27+3+2+25+5=62
62/5=12.4
Final answer: D, 12.4 years