Dat be called common difference represented by d in the equation
the answer is 3
but a way to work these problems out is to use PEMDAS
p-parentheses
e-exponent
m-multiplication
d-division
a-addition
s-subtraction
they have to be donr in that order or the answer will come out different
If you're solving for a, you would add 32 to both sides
2a=32
then you would divide by 2
a=16
If you need the sum than here is an equation for it 7x3= 21