The line AB has equation 3x + 5y = 7 (a. Find the gradient of AB)

This is an easy one as long as you remember the important equation y = mx + c. I can't express how important it is for you to learn that equation.

Step one: find y. Do this by taking 3x away from both sides and dividing by 5 so that the y is by itself. y= 7/5 - 3x/5

Step two: identify that this looks like your equation y=mx+c where m is the gradient and c is a number.

You therefore know that the gradient of AB is -3/5.