How do i know if its a linear function and decreasing

You would know if it is a linear function if it is a line.

The parent function of a linear equation is y = mx + b

If you can find the m which equals slope and the b which is the y-intercept then it is a linear equation.

You would know that it is decreasing by the slope of the equation or by looking at the line on a graph.

If a line is increasing it would go left to right in the positive direction. But in your line, the line is going from right to left which means that it is decreasing. Once you look at a four quadrant graph you would understand that there are negatives and positives.

To find it on a equation, your slope would have to be a negative slope to be decreasing.  

ab will be equal to