Volume of the block is x(12x-15)(12x-21). This is from one side. From the other side we know that this volume is equal to 945.
Now we "equal both sides":
x(12x-15)(12x-21)=945 [(12x-15=3(4x-5) and 12x-21=3(4x-7) ]
9x(4x-5)(4x-7)=945 (dived both sides by 9)
Since 16x2+35≠0 for x from real numbers, then x=3.
So, Height: x=3
Width: 12x-21= 36-21=15
Check: 3*21*15= 945