First of all, we know that Logan's County has the shaped of square, and the question states that to find the area of his county. To find the area, we will use Area of square formula.
A=s•s where s represent the side of his county which is 15.4 miles long, so we just beed to do now is substitute s with 15.4
A=s•s
A=15.4•15.4
A=237.16 square miles. As a result, the area of Logan's County is 237.16 square miles. Hope it help!