Maths  Properties of Planes
posted by Shaila .
write a scalar equation, given it's vector equation.
[x,y,z]= [3,7,5] + s[1,2,1] + t[5,2,3]
( I remember my teacher saying that you have to cross s and t, and that will be my normal and thereafter I substitue the point to find the 'D' value of the scalar equation, but when I try it, it does not give me the answer? Is my method wrong?)

did you get a reduced crossproduct of (1,2,3) ?
Here is an online crossproduct calculator:
http://www.analyzemath.com/vector_calculators/vector_cross_product.html
so your equation would be x  2y  3z = D
sub in your given point and you got it.