@dattasai and @Taiyaba Khatoon
'Macro expansion' means replacing a macro call in a program, by the definition of that macro.
Here, the macro call hypotenuse(a+2,b+3), wherever it appears in the program, is replaced by sqrt(a+2*a+2+b+3*b+3)
* has higher precedence than +, so the expression evaluates to $a + 2a + 2 + b + 3b + 3 = 3a + 4b +5$, instead of $\left ( a+2 \right )^{2} + \left ( b+3 \right )^{2}$