Meters for mass kilograms for volume cubic meters for density kilograms per cubic meter
Since there is no decimal point in the number given above, the counting for the number of the significant figures will start from the left. Then, the first zero from the left is insignificant. Therefore, in this number there are 6 significant figures.
15 = a+(3-1)d
23 = a+(7-1)d
simultaneous eqs. solve for a and d