Question

y=a⋅xb
whose graph passes through (3, 4) and (6, 7)

Answer 1

If you meant what you wrote, then
a*3b = 3
a*6b = 7
divide and you have 2 = 4

I suspect you meant wither
y = a * x^b
or y = a + x*b
or something else.

Answer 2

If your question means:

y = a x - b

then

Equation of a straight line in two point form:

y - y1 = ( y2 - y1 ) ( x - x1 ) / ( x2 - x 1 )

In this case:

x1 = 3 , y1 = 4 , x2 = 6 , y2 = 7

y - 4 = ( 7 - 4 ) ( x - 3 ) / ( 6 - 3 )

y - 4 = 3 ( x - 3 ) / 3

y - 4 = x - 3

Add 4 to both sides

y = x + 1