A mass m is attached to the end of a spring whose constant is k. After the mass reaches equilibrium, its support begins to oscillate vertically about a horizontal line L according to a formula $h(t)$. The value of h represents the distance in feet measured from L. Determine the DE of motion if the entire system moves through a medium offering a damping force that is numerically equal to (beta: B)(dx/dt).
I understand how to set up the equation and to solve when given numerical values for the above variables. I am stuck however with their solution. They are multiplying h(t) by the spring constant to get the DE:
$$mx''(t) = -kx(t) - Bx'(t) + kh.$$
I thought it would be $$mx''(t) = -kx(t) - Bx'(t) + h(t)$$ and don't understand why we multiply by the spring constant for the $h(t)$ term.
Can anyone please help me understand this part?