Example 1.   Work out the value of a in 2⁴ = 4^a

2⁴ = 2 × 2 × 2 × 2 = 16
4² = 4       × 4       = 16

2⁴ = 4² = 16
so
a = 2

Example 2.   Work out the value of a in 3^a = 9²

3⁴ = 3 × 3 × 3 × 3 = 81
9² = 9       × 9       = 81

3⁴ = 9² = 81
so
a = 4

Example 3.   Work out the value of a in a⁴ = 4²

2⁴ = 2 × 2 × 2 × 2 = 16
4² = 4       × 4       = 16

2⁴ = 4² = 16
so
a = 2