To find the reflection of the function \( f(x) = -7x - 3 \) across the y-axis, we need to replace \( x \) with \( -x \) in the function.
Starting with \( f(x) \):
\[ f(x) = -7x - 3 \]
Now, we replace \( x \) with \( -x \):
\[ g(x) = f(-x) = -7(-x) - 3 \]
This simplifies to:
\[ g(x) = 7x - 3 \]
So, the reflection across the y-axis is:
\[ g(x) = 7x - 3 \]
In the form \( mx + b \), we have \( m = 7 \) and \( b = -3 \).
Thus, the final answer is:
\[ g(x) = 7x - 3 \]