In a quadratic equation, the variable x must be raised ti a power of two, this is not our case, since x is not raised to any power.
Linear equations has the form:
y=mx+b, let's arrange our equation to see if we have a linear equation
x-3y=15
by adding 3y in both sides:
x-3y+3y=15+3y
we can cancel 3y in the left side, then we have:
x=15+3y
subtracting 15 from both sides
x-15=3y
dividing by 3 in both sides:
as we can see, our equation fits into the form of linear equation y=mx+b, where m=1/3 and b= -5