the product of slope of two mutually perpendicular lines is -1.

but the product of slope of X-axis and Y-axis is not -1.

hey sachit 

If the products of slopes of two mutually perpendicular lines is always -1

i.e. if two mutually perpendicular lines have slopes m₁and m₂ respectively then

i think this figure will give you the answer for the solution you are loooking for 

Because the slope of the y axis is actually undefined, so it's questionable whether one can even talk about the product of these slopes.