So you need to solve for p and q, using solution addition/subtraction.
for this particular question I will subtract. p+q=4 from 4p+q=1
4p+q=1
-(p+q)=4
_________
3p +0q = -3
3p=-3
3/3p=-3/3
p=-1
With the value of p found insert p into one of the original equations
--> Usually choosing the easier one is easier
p+q=4 Inset p=-1
-1+q=4
q=4+1
q=5
Therefore,
q=5, p=-1
