Answer:
The general equation of the lines that are perpendicular is:

b can by any number.
Step-by-step explanation:
The equation of a line is given by:

where m is the slope and b is the intercept
The equation of a perpendicular line to the previuos one is:

That means that the slope of a perpendicular line is inverse and sign opposite to the other line.
Hence, the equation given is:

m would be:
m=1
So the perpendicular slope would be:

The general equation of the lines that are perpendicular to y=x+3 is:

b can be any number
In attached picture:
y=x+3 is the red line
y=-x+b is the blue line