Question 1077822
Let the altitude = {{{ a }}} cm
Let the base = {{{ b }}} cm
------------------------
(1) {{{ a = b - .2b }}}
(1) {{{ a = .8b }}}
and
(2) {{{ 2a + 2b = 18 }}} 
----------------------
(2) {{{ a + b = 9 }}}
Plug (1) into (2)
(2) {{{ .8b + b = 9 }}}
(2) {{{ 1.8b = 9 }}}
(2) {{{ b = 5 }}}
and
(1) {{{ a = .8b }}}
(1) {{{ a = .8*5 }}}
(1) {{{ a = 4 }}}
The rectangle is 4 x 5
-----------------------
check:
(2) {{{ 2a + 2b = 18 }}} 
(2) {{{ 2*4 + 2*5 = 18 }}} 
(2) {{{ 8 + 10 = 18 }}}
(2) {{{ 18 = 18 }}}
OK