Consider the following production function: Q = K^a + KL^(b+2), find the marginal product of capital.

The marginal product of capital (MPK) is defined as the additional unit of output, all things being equal, that comes with an additional unit of capital used.
The equation for MPK is: change in output/change in capital.
This would look like:
MPK = (a)(K^a-1) + (1)(K⁰)(L^b+2) = aK^a-1 + L^B+2