Final answer:
The dimension of matrix b is 5x2.
Step-by-step explanation:
The dimension of matrix b can be determined by considering the dimensions of the matrices involved in the addition operation a + b. In order for matrix addition to be possible, the matrices being added must have the same dimensions. Since matrix a is given as a 5x2 matrix, in order to compute the sum a + b, the matrix b must also have dimensions of 5x2.