Question

A=2b^3-1 what's the value of b if a is 13

Answer 1

$\sf~A=2b^3-1$

Plug in A = 13

$\sf~13=2b^3-1$

Add 1 to both sides:

$\sf~14=2b^3$

Divide 2 to both sides:

$\sf~7=b^3$

Find the cube root of both sides:

$\sf~b=\sqrt[3]{7}$

$\sf~b\approx~1.91$

Answer 2

A = 2b³ - 1
13 = 2b³ - 1
+ 1         + 1
14 = 2b³
2       2
7 = b³
∛(7) = b