Upon receipt of the petition, findings and recommendations the state department of revenue shall proceed to consider the same, and it may require evidence to be submitted and make such investigation as it deems necessary and for such purpose the department of revenue shall be empowered to subpoena witnesses in order that all material and relevant facts may be ascertained. Upon the conclusion of its consideration of the petition and within thirty days after receipt thereof, the department of revenue shall enter an order either granting or denying the petition and if the petition be granted the department of revenue may order the assessment canceled or reduced or the extended tax corrected upon the tax rolls in any amount it deems proper but in no event to exceed the amount of reduction or correction recommended by the county assessor.