deduce the equation of a line which passes through (2,1) which makes an angle 45 degree with positive direction of x axis

The line passes through (2,1) and makes 45 degrees with x axis.
=> Slope of the line = tan(45) = 1
=> Equation of line is,
y-1=1(x-2)
y-1=x-2
x-y-1=0 is the required equation.
Hope this helps! :)