The first thing I would probably do is distribute (multiply) the -5/2 into the parenthesis:
= (-5/2)x + (-20/2)
Notice how I am not putting the x into the fraction. This is to make it easier to see the slope-intercept form.
Lets simplify the (-20/2) and then add 7 to both sides:
y - 7 = (-5/2)x - 10
y = (-5/2)x - 3
Your answer would then be:
y = (-5/2)x - 3
y = mx + b
Since it is now in the requested format.