Take any point of the line. I'm going to use (0,2).
(x,y)=(0,2). Therefore x=0 and y=2.
Next, b is the y intercept, and in my explanation (0,2) is the y intercept. Therefore b=2.
Now substitute variables for numbers that you know.
2=0m+2
Therefore m=1.
Now finally substitute m and b into your equation.
So your equation is:
y=x+2