Answer:
Given the equation: \(d = \frac{LM}{R_2+R_1}\) .....[1]
Cross multiply states that an equation of fractions when each of the side consists of a fraction with a single denominator by multiplying the numerator of each side by the denominator of the other side and equating these two products obtained.
Apply the cross multiply in [1], we get;
\(d(R_2 +R_1) = LM\)
Divide both sides by M we get;
\(L = \frac{d(R_2+R_1)}{M}\)
or
\(L = \frac{dR_2+dR_1}{M}\)