12+8k=48
=>8k=36
=>k=36/8
=>k=4.5
Sample space = HHH, HHT, HTH, THH, TTT, TTH, THT, HTT (8 possibilities)
Desirable outcomes = TTH, THT, TTH (3 possibilities)
Required probability = 3/8
Substitution is best to use one one or both equations is already solved for one of the variables. Elimitation can be use if all variables have a coefficient other than 1.