Let the total cost of the car be \( x \).
According to the problem:
- Brian pays \( \frac{1}{4}x \)
- Don pays \( \frac{3}{8}x \)
The sum of their contributions and the loan from Don's mother should equal the total cost of the car:
\[ \frac{1}{4}x + \frac{3}{8}x + 300 = x \]
To combine the fractions, we first need a common denominator. The least common multiple of 4 and 8 is 8. We can rewrite \( \frac{1}{4}x \) in terms of eighths:
\[ \frac{1}{4}x = \frac{2}{8}x \]
Now substituting back into the equation, we get:
\[ \frac{2}{8}x + \frac{3}{8}x + 300 = x \]
Combining the fractions on the left side:
\[ \frac{2 + 3}{8}x + 300 = x \]
This simplifies to:
\[ \frac{5}{8}x + 300 = x \]
To isolate \( x \), we subtract \( \frac{5}{8}x \) from both sides:
\[ 300 = x - \frac{5}{8}x \]
This simplifies to:
\[ 300 = \frac{3}{8}x \]
Now, to find \( x \), we multiply both sides by \( \frac{8}{3} \):
\[ x = 300 \cdot \frac{8}{3} \]
Calculating this gives:
\[ x = 300 \cdot \frac{8}{3} = 100 \cdot 8 = 800 \]
Thus, the total cost of the car is \( \boxed{800} \).