Question 6381
Recall that perpendicular lines have slopes that are the negative reciprocal of each other.

What does this mean?

Well...if you know the slope of a line, then you automatically know the slope of all lines that are perpendicular to it.  For example:  If the slope of a line is m = (-3/5), then the slope of the perpendicular would be the negative reciprocal of (-3/5) which is +5/3

So, the given line has the slope, m = 1.

How do you know this?  The given equation is in the slope-intercept form:
 y = mx + b Compare this with:
 y =  x + 1 and you'll see that m (the slope) is 1.

Now your problem doesn't state which form of the equation is required so you could use the slope-intercept form or the point-slope form. 
The point-slope form looks like this: y - y1 = m(x - x1) and this would be easier to write since you have all of the required information at hand and no calculations are necessary.  The x1 and y1 are just the x and y values of the given point (1, 3)

Now, what is the slope of the new line?  It's just the negative reciprocal of 1 which is -1.

Now we can write the equation of the line (in the point-slope form) that is perpendicular to given line (y = x + 1) and which passes through the given point (1, 3).

y - 3 = -1(x - 1)