Linear programming problem

Would very much appreciate some assistance with a Linear modelling problem. Facts: 2 production lines, 2 products, 8 hours per line (480 min), profit for each product, a is $42, b is $87

Line 1 Line 2

Product a 3 minutes 4 min

Product b 6 minutes 2 min

Maximize profit formula: 42a + 87b

Constraints: 3a + 6b<=480

4a + 2b<=480

I'm trying to determine the additional constraint that reflects the question...

how can we limit the difference between the total time on line 1 and the total time on line 2 to 30 minutes or less...in order to maximize profit? Thanks.