look at the equations:
x + ay = 5
2x + 6y = b
it looks like the 2nd one is twice the 1st, but be careful... if they are the SAME LINE then all real numbers are solutions! ! So it can't be A)
We only want them to intersect ONCE for one solution, so plug it until they have different slopes:
in B) and C) there is no solution when you plug in because the x's and y's cancel each other out, so a cannot = 3
Therefore the only option is D) a=10, b=3