1. fine

2. Not clear if you mean "short run total variable cost" or "short run average variable cost".

SRTVC = 15L

SRAVC = 15L / 10

3.

Compute the amount of labour needed to produce X units of output given that K is fixed at 4.

Compute the SRTC = 15L + 4*5 [[note: it will be a function of x!!]]

Differenciate SRTC with respect to X to obtain the SRMC