Addition inside of an existing Matrix

I am trying to understand how to manipulate a matrix, can anyone please show me how to perform the following action in Matlab?

for example if I had a 2x2 matrix

[a,b;c,d]

How would I be able to achieve

[a,b;c,x+d]

e.g a=1, b=2, c=3, d=4,

if X=10

[1,2;3,X+5]

Answer 1

[matico]

M = [1, 2; 3, 4];
X = 10;

%%VERSION 1

M(end) = M(end) + X;

%%VERSION 2

M(2,2) = M(2,2) + X;

In both versions the M matrix become [1, 2; 3, 14]