When a function is reflected across the x-axis, the y-coordinate is transformed into its opposite sign. Basically you move the graph of the function across the x-axis to its mirror reflection, hence for the function f(x) = -5x + 2 the reflection across the x-axis is;
f(g) = -5x - 2
For the function f(x) = -3 + x - 11, the reflection across the y-axis is;
f(g) = - x - 14