Recall that the formula for the perimeter of a rectangle is: 2Length+2Width
With this in mind, we have that the original design is 96×60, where 90 is length and 60 is width, so it would simply be:
2(96+l) + 2 (60+w) = New Perimeter
Or simplified, it'd be:
2l + 2w + 312 = New Perimeter