OK, for starters, this: " I thought you multiply 0.926 three times so you can multiply the result of 0.926 three times with 3.5 and then from that result, you use a calculator to find the cubic root, then add 0.169 but I don't get 1.237 mm as the answer. " makes no sense. You follow the order of operations-- do the exponent (cube root) first, then multiplication, then addition/subtraction. I think you copied the formula wrong. Either that, or there's a typo somewhere-- check other similar problems and see whether the constant should be positive or negative. If you change the sign of the 0.169 to a minus, it works out. Here's what I get: 0.926 (3.5)^1/3 + 0.169 0.926(1.518) +0.169 (I'm assuming you want 3 decimal place) 1.406 -0.169 1.237 And I'm not a math genius; I've just been doing this forever.