Answer:
b=M/(4+9x)
Explanation:
In order to solve this you first have to factorize the b from the equation:
M = 4 b + 9 b x
M = B(4+ 9x)
As you can se ethe B is multiplying the factor (4+9x) so in order to clear B (4+9x) should go to the other side dividing M:
B=M/(4+9x)
And that would be the answer to solving the equation for B.