See image below. A three$-$span one$-$way concrete slab is subjected to dead load $6k\frac{N}{m^2}($self$-$weight included$)$ and live load $3k\frac{N}{m^2}.$ Concrete slab thickness $=200mm,L=5m,$ and support width $=300mm.$ The Young's modulus of concrete $=30$GPa and the flexural reinforcement is N$12$@$250$ cts for the entire slab. Determine the required effective depth $($unit: mm$,$ round to a whole number$)$