The coordinates of three points are A(-1,-3), B (2,3) and C(6,k) If AB is perpendicular to BC then find a) The value of k
b) the gradient of AC
c) the acute angle that AC makes with the x-axis.
My answer for (b) is 4/7 but the actual answer is 4/5. And I have problems with (c) too. Kindly please show your working. Thank you very much.

Plz answer asap! i will reward brainliest and 12 points for these three questions!take notice: these three questions are completely different! 1) what is the percentage of sugar in the syrup made of 10 kg of water and 4 kg of suugar? 2) what was the original price if after 10% discount it became $450? 3) at first a price was decreased by 12%, then it was disounted again by an additional 5%. what is the percent of the total discount? , and remeber i will award brainliest and 12 points!

What is the surface area of a cube with the volume of 729 ft?

Polygon abcd is a parallelogram, and m? abc = 127°. the length of is 10 units, and the length of is 5 units. the perimeter of the parallelogram is units, and m? bcd is °