calculate change in power of the arc w.r.to change in arc length

Question

In MIG welding the power source characteristics is V_p=36-I/60 and the arc characteristics is V_a=2L_a + 27. find the change in power of the arc if the arc length changed from 2mm to 4mm. If the maximum current capacity of the power source is 300 Amps, the maximum arc length sustainable is