The speed v of a wave on a stretched string depends on the tension F and the mass per metre µ.

Step 1. Powers. $[\text{L}\,\text{T}^{-1}] = [\text{M}\,\text{L}\,\text{T}^{-2}]^{x}[\text{M}\,\text{L}^{-1}]^{y}$. T: $x = \frac{1}{2}$. M: $y = -\frac{1}{2}$. L: $x - y = 1$, which checks.

Step 2. Form. $v = k\sqrt{F/\mu}$.

Step 3. (b). $v = \sqrt{\dfrac{90\ \text{N}}{0.010\ \text{kg/m}}} = 94.9\ \text{m/s}$.